If you’re familiar with the Rose Rule and don’t need a breakdown of how it works feel free to skip to the picture of Derrick Rose.
After a player’s rookie contract runs out they become a “restricted free-agent” and enter the open market. Any NBA team with enough cap room is free to offer them a deal, with the caveat that being “restricted” means as long as the team which drafted the player is willing to match the outside offer, they get to keep him regardless.
A max contract for a 4th year player is 25% of the cap.