Per the latest CBA players gain UFA status after 4 years experience. AKA every player drafted who plays out their first contract.
RFA means 3 years of experience. Usually you don't see too many players hit RFA status unless they were undrafted or drafted, cut, passed waivers and then signed 1 to 2 year deals.
For example David Irving was a RFA in the 2018 offseason as he had 3 years experience last year and his contract expired. The Cowboys had to send him a qualifying offer in order to retain his RFA status or he would of hit the market as an UFA.
There are 3 Qualifying offers. 1st round tender, which pays the most, but means if a team signed Irving to an offer sheet, and the Cowboys declined, they would send us a first in order to officially sign him. Then there is a 2nd round tender, same as before, and an original drafted round tender. IE if the player was originally taken in the 4th, that's the pick the other team had to surrender to sign him.
Basically, NFL teams have rights over players till 4 years of experience is accrued, then they are unrestricted and the only method of the team retaining the player is the franchise tag or transition tag.
There's 3 free agent statuses in the NFL.
ERFA: 1 to 2 years of experience(when the contract expires). Can't negotiate with other teams if qualified.
RFA: 3 years experience... . Can negotiate with other teams if qualified.
UFA: 4+ experience. Free to talk to any team if no deal is reached.