Two words keep popping up in baseball news these days: free agent.

That’s because two stars from the Washington Nationals’ championship team — pitcher Stephen Strasburg and third baseman Anthony Rendon — became free agents at the end of the season.

That means Strasburg and Rendon were no longer required to play for the Nationals. They could choose to sign another contract to play with Washington, but they were also free to sign a contract and play for a different team.

With baseball contracts, a player promises to play for a certain number of years, and the team promises to pay him a certain amount of money. Dozens of major league baseball players are free agents and may move to other teams over the winter.

This week, Strasburg decided to sign a contract with Washington. Under the contract, he will pitch for the Nats for seven more seasons.

Recently, Mark Lerner, one of the Nats’ owners, said the team may be able to keep only one of their star free agents. So Rendon may play for another team next season.

For much of the history of professional baseball, there was no such thing as a free agent. Players signed contracts with something called a “reserve clause.” This reserve clause meant the player was bound to one team.

Teams could trade players, but the players could not choose to play for another team and move without their team’s permission.

The owners liked the reserve clause, because they could keep their best players. The reserve clause also helped to keep the players’ salaries low, because other teams could not offer players more money to switch teams.

In the late 1960s, players, including three-time all-star Curt Flood, challenged the reserve clause. They did not think it was right that players had no say in where they played. After all, other workers, such as doctors and teachers, can decide where they want to work.

After years of legal battles, a panel decided in December 1975 that the reserve clause could not keep a player on a team forever. Now, if a player has been in the major leagues for a certain number of years and he is at the end of his contract with a team, he is free to sign with another team.

As a result, more players move from team to team. And teams, such as the Nationals, can have trouble keeping their players.

In addition, salaries for major league players have soared. The average major leaguer in 2019 made more than $4 million (a middle-income American worker makes about $60,000). It has been reported that Strasburg will make $35 million a season under his new contract.

They are just two words, but those words — free agent — are a big deal in baseball.