RESTRICTED FREE AGENT:
A Restricted Free Agent is a Player that has had their salary expire at the end of a season.  This player may be retained in free agency if the owner matches this winning bid.
Iron Salary Page

Roster limits
Franchise Salary Player (do not expire)
  12 years @ cost of 12 FDs
  (min 1 player max 1 player)
5 Year Salary Players @ cost of 5 FDs 
  (min 0 max 1 players) 
4 Year Salary Players @ cost of 4 FDs
 (min 0 max 3 players) 
3 Year Salary Players @ cost of 3 FDs
  (min 0 players)
2 Year Salary Players @ cost of 2 FDs    (min 0 players)
1 Year Salary Players @ cost of 1 FD   (min 7 players)