For lack of better words, a restricted free agent is a protected player. While not the perfect description its the best way to initiate the conversation regarding RFAs.
The RFA process is quite difficult and tedious to understand. It’s a process that’s strictly regulated by the NHLs collective bargaining agreement. If you want to spend a good portion of your day looking over some very complex legalese here is the CBA in all it’s glory: NHL CBA
The idea behind RFAs is to give clubs the opportunity to keep their young players instead of having them flee to bigger markets.