The Collective Bargaining Agreement between the NFL and the Players Association is set to expire after the 2010 season. As an incentive to spur extension talks the 2010 season is uncapped. Most owners don't want an upcapped league, and the players association doesn't drool over the prospect since it would also remove the salary floor, allowing teams to spend as little as they want on their roster, meaning an uncapped league could very well not increase, or could even decrease the money going to players.

In the event of an uncapped season, the league didn't want all the richer teams to buy up all the talent, so there are added restrictions on free agency in an uncapped season, by far the most relevant of which being an increase in service time requirements for unrestricted free agency.

ERFA (Definition from AsktheCommish)

may only sign with his prior team, provided, of course, that the team extends a minimum qualifying offer (league minimum) to the player.

RFA-Teams can offer a tender to their RFAs which grants them "right of first refusal" (the ability to match any contract offered to the RFA) and draft pick compensation from the signing team according to the level of tender offered to the player.

UFA-Player can sign with any team.

So the difference between free agency in an uncapped and capped season is players with 4 years of service, are RFAs instead of UFAs. The Colts have 4 players who's free agency status will depend on whether a new/extended CBA is signed,

Antoine Bethea, Tim Jennings, T.J. Rushing and Charlie Johnson.

While the lack of a deal on the CBA would remove the cap, possibily never to be re-applied and be a major step towards a work stoppage, it would make it much easier and cheaper for Indy to keep 4 key role players.