At 11:59 p.m. Eastern on Wednesday, the document that governs baseball — the five-year collective bargaining agreement between the owners of the 30 Major League Baseball clubs and the players — expired. Two minutes later, M.L.B. Commissioner Rob Manfred announced that the league had enacted a lockout, which brought the sport to a halt. Teams are not allowed to talk to players, make major-league signings or swing trades.

This is the ninth work stoppage in M.L.B. history, and the fourth lockout. Here is what’s going on:

What is a lockout?

In short, it is a type of work stoppage used by the owners of a business during a labor dispute.

“In a lockout, the management tells workers don’t show up,” said Bob Jarvis, a professor who teaches a baseball law course at Nova Southeastern University. “While in a strike, workers tell management, ‘We’re not going to show up.’ But either way, the factory or the business comes to a standstill because the workers are not there.”

A lockout is a tool that owners can use. Leagues can operate without a new collective bargaining agreement in place, but it is common practice in the four major men’s North American professional sports leagues — M.L.B., the N.F.L., the N.B.A. and the N.H.L. — to use a lockout in instances like this.

Manfred referred to M.L.B.’s implementation of the lockout as “defensive.” His reasoning: the 1994-95 strike — the last work stoppage in baseball to cost regular-season games (over 900 in all) — came after the league had continued to operate without a new agreement.

When the C.B.A. expired then, the 1994 season began as scheduled. Players went on strike in August as M.L.B. sought to add a salary cap. The union successfully avoided the institution of the cap, but the 1994 World Series was canceled and the strike didn’t end until April 1995, with a legal battle playing out in public.

“You want to control the timing of the dispute,” Manfred said, adding later, “If you play without an agreement, you’re vulnerable to a strike at any point in time. What happened in 1994 is the M.L.B.P.A. picked August when we were most vulnerable because of the proximity of the large revenue dollars associated with the postseason. We wanted to take that option away and try to force the parties to deal with the issues and get an agreement now.”

Players felt as if owners had been planning a lockout for some time and that it was an attempt at intimidation.

“It’s unnecessary to continue the dialogue,” said Tony Clark, the executive director of the players’ union. “We’ve obviously had 26 years without a work stoppage and the industry has continued to do well and grow. And at the first instance in some time of bumpy water, the recourse was a strategic decision to lock players out.”

How many lockouts have led to the cancellations of games?

None yet.

In the lockouts of 1973, ’76 and ’90, a full regular-season was played each time. In 1990, for instance, the 32-day lockout eliminated most of spring training, but 162 regular-season games were played, starting a week later than usual.

What are the downsides of a lockout?

Even though it is the off-season, Manfred conceded that a lockout is “bad for our business.” No games or paychecks are being missed over the winter, but Clark said players considered the lockout “provocative.”

“The moment that you declare a lockout, you really push fans into the players’ camp,” Jarvis said. “You really play into the players’ argument that, ‘Hey, even though we’re the reason that you’re coming out to see the games, you won’t be able to see the games because of what management did.’ It is much better for management to force the players to go on strike so that management can say, ‘Hey, we want the games to go on. It’s the players who are refusing to show up.’”

Jarvis said that, at least, this work stoppage was coming during a time when other sports are in season: the N.B.A., the N.F.L., the N.H.L., and college football. But for ardent baseball fans who intensely follow their team’s hot stove moves, he said a lockout is bad because the activity has stopped. For the average fan, he said, a lockout isn’t going to make a difference. That is, until spring training. If a new agreement is not struck by then, the casual fan will start to notice when the usual schedule is altered. More will when the regular season is affected.

“Right now, the calendar favors ownership,” said Michael LeRoy, a professor and sports labor expert at the University or Illinois at Urbana-Champaign. “Players can’t sign free-agent deals and they’re going to get anxious about that. But if players are able to hold fast through late January or February, now the advantage shifts to the players. It’s not very common, but there are times that lockouts convert to strikes. My concern about this is that a lockout would not be resolved quickly, and that in time it would morph into a strike and it would last into spring training and beyond.”

LeRoy said there have been roughly 20 work stoppages in the four major North American sports leagues since the 1960s with almost all of them won by management. The lone exception: the 1994-95 baseball players’ strike.

“That was the strike that set the bar for labor unions in sports to emulate and so that is framing everything here,” he said. “That was a spectacular success for the union that was so successful, because the union avoided a hard salary cap. And that’s why we have players signing 10-year deals for over $300 million. You have nothing like it in any other sport.”

What are the issues in the dispute?

There are a lot.

Although star players are setting contract records in a system without a hard salary cap — a mechanism present in the other major North American professional sports leagues — players feel owners aren’t struggling as much as they say they are; that too many teams are receiving tens of millions in revenue sharing from their counterparts yet purposefully aren’t competing for playoff spots; that the industry has grown but the average major-league salary (roughly $4 million) has remained flat or dropped; that younger, cheaper players are being relied on more than ever and having their service time manipulated.

Owners, though, believe baseball players have the best deal in professional sports and point to this off-season’s free-agent spending, which was on pace to set a record, as one point in that argument.

Read more about their dispute here, here, here and here.

What can players do during a lockout?

They cannot sign major league contracts. They cannot take part in team activities. They should not be talking to team officials. They cannot use team facilities. (A fair amount of players, though, already work out with private trainers or coaches in the off-season.)

Some of the specifics of what players can do seems to be in dispute. Asked if contact between club officials and players undergoing rehabilitation for injuries or those talking to team mental health professionals could continue, Manfred said, “That’s a legal issue upon which we do not have flexibility.”

Clark countered by saying “We have a difference of opinion in regard to what the rules of engagement are here. There have been differences in other scenarios whereby support was provided to those who needed it. So on that front, it is not a line in the sand that is necessary to draw.”

On Thursday morning, Yankees pitcher Jameson Taillon, who is recovering from ankle surgery in October, tweeted that he was going to have to find his own way to continue his rehabilitation.

“Since MLB chose to lock us out, i’m not able to work with our amazing team Physical Therapists who have been leading my post surgery care/progression,” he wrote. “Now that I’m in charge of my own PT- what should my first order of business be? I’m thinking I’m done with this boot. It can go.”

The union has told players that, if they were on a 40-man roster at any point during the 2021 season, their medical benefits will continue until the scheduled start of the 2022 season, according to a work stoppage guide provided to agents, a copy of which was obtained by The New York Times. If a work stoppage continues after then, the union said it will use its reserves to cover their health benefits.

What can clubs do during a lockout?

Teams were expected to continue their off-seasons but without a major component: the players. They can work on trades but cannot consummate them. They can continue working with their non playing employees, scouring video, planning their strategies and so on.

Will club employees be laid off or furloughed?

An M.L.B. spokesman said that “for the foreseeable future, there are no plans at the league or club level to implement furloughs, pay reductions or work force reductions” while M.L.B. negotiated what it hoped would be “a full 2022 season” with the union.

In 2020, when the pandemic halted everything in spring training and negotiations over a regular season lasted months, many clubs furloughed or laid off employees, and instituted pay cuts.

Can the sides still negotiate during a lockout?

Yes. They said they planned to, even though no further sessions had been scheduled as of Thursday morning.

“There’s this intense negotiation and then when the lockout occurs, there’s emotion around it and people back away,” LeRoy said. “And usually there’s a fairly lengthy hiatus before any more talks occur. It wouldn’t surprise me if it was weeks later that talks occur. And really, what it becomes is a war of attrition. Who can endure the pain the longest?”

After the C.B.A. expired, neither Manfred nor Clark provided a deadline for a deal. Manfred said M.L.B. enacted the lockout now to provide enough time to reach a resolution that avoids damaging the 2022 season. Clark said players want an agreement that addresses their concerns and hoped it would be “sooner rather than later.”

Is anyone missing a paycheck?

Not yet.

Players don’t earn paychecks or service time in the off-season or spring training, during which they get allowances. Paychecks and service time start in the regular season, which is slated to begin on March 31. (During a lockout, according to the union, clubs are obligated to pay any signing bonuses, deferred salary and any other payments that were earned before the lockout began.)