There are a few ways that one can structure contracts, and some players have player options/team options as part of their deals. A team option in a given contract year would allow the team control of whether they want to keep the player on the roster for another year, while a player option would give that same control to the player, allowing them to choose to remain on the team or become a free agent.
There have recently been two NBA All-Stars that have rejected lucrative player options in order to become free agents. The first of those two All-Stars is Chris Paul, who rejected a $44.2 million player option. Paul had a fantastic season for the Phoenix Suns, where he played a key role in their run to the NBA Finals.
The second player is Kawhi Leonard, former Finals MVP, and superstar on the Los Angeles Clippers. Kawhi Leonard declined a 36 million player option to become a free agent, and could potentially sign a new deal with the Los Angeles Clippers or elsewhere.
It has to be said that while Chris Paul and Kawhi Leonard will become free agents with this move, that doesn't necessarily mean that they will leave their teams. Oftentimes players decline player options to potentially sign new lucrative deals with their current franchises, and it seems as though there is optimism from the Phoenix Suns that Chris Paul will take that route.
While there has been no immediate reporting on whether Kawhi Leonard will sign a new long-term deal with the Los Angeles Clippers, a prior report from Adrian Wojnarowski suggests that he will do exactly that. However, things in the NBA could change in an instant. It remains to be seen whether the two All-Stars entertain offers from other teams, and there is no doubt that there would likely be a market for both of them. However it is entirely possible that they will re-sign with their teams as well, and we will see what they end up doing once free agency officially opens up.