This is from nfl.com website...
Q. Other than accrued seasons, what determines a restricted free agent?
A. He has received a "qualifying" offer (a salary level predetermined by the Collective Bargaining Agreement between the league and its players) from his old club. He can negotiate with any club through April 15. If the restricted free agent accepts an offer sheet from a new club, his old club can match the offer and retain him because it has the "right of first refusal." If the old club does not match the offer, it can possibly receive draft-choice compensation depending on the amount of its qualifying offer. If an offer sheet is not executed, and the player receives the June 1 tender from his old club, the player's rights revert exclusively to his old club on June 1.