In the NBA, there are two types of free agency: unrestricted and restricted.
Players are eligible for restricted free agency once their rookie deals expire. In order for a team to make a player a restricted free agent, it must offer the player a qualifying offer before the July moratorium. A qualifying offer is a one-year contract with a value that is determined by the player's previous salary. If a team decides not to give a qualifying offer, the player becomes an unrestricted free agent (generally if a player is good, this does not happen).
This year's numbers are determined by the 2015-16 Rookie Scale.