A more thorough explanation:

A reserve clause is a provision in a professional athlete's contract that limits the athlete's ability to switch teams, even after the contract has ended. This means that the athlete is bound to the team for a certain period of time, and cannot leave or join another team without the team's permission.

For example, in the early days of Major League Baseball, the reserve clause was used to keep players tied to their teams for their entire careers. This meant that players had little bargaining power and were often paid very little compared to their value. It wasn't until the 1970s that the reserve clause was challenged and eventually abolished, leading to the rise of free agency.

Today, reserve clauses are rare in professional sports, as most leagues have adopted free agency or other forms of player movement. However, some contracts may still include limited reserve clauses that restrict a player's ability to leave the team before a certain date or under certain conditions.