There are two criteria required to describe a restricted free agent (RFA) in the NFL. First the player must have played three seasons and their contract has expired (a season is defined by 6 or more regular season games played). The second step is that the player then receives a qualifying offer from his team, "qualifying offers" are a predetermined dollar amount that was negotiated in the Collective Bargaining Agreement).
If those two conditions are met, the player is considered a restricted free agent. They are able to negotiate a new contract with any other team in the NFL up until April 15th, upon acceptance of an offer with a new team, the player's original team has the right of first refusal to match the offer. If matched, the player remains with the same team under the contract terms negotiated with the potential suitor. Should the player's original team decide not to match the offer, then the player joins the new team.
The difference between a restricted and unrestricted free agent is years of service in the NFL. An unrestricted free agent is someone who has played four or more seasons and whose contract is now expired, they are free to sign with any team with no opportunity for the original club to match the offer.