For players who didn’t finish the 2017 NFL season under contract with an NFL team, which includes players who finished the year on practice squads, these guys are free at any point after the season is over to sign what’s called a “reserve-futures deal.”
These contracts don’t count against the salary cap or 90-man limit until the following league year, which for 2018 begins on March 14.
These contracts just basically claim the rights of that player to that team.
This was the case a few years ago with running back Spencer Ware, who was out of football in 2014 and then signed a reserve-futures deal with the Chiefs after that season.