The jail is where your spartan gets sent to by scouting turns, such as getting caught while stealing supplies. Depending on the scouting turn you may be in jail for 3 minutes, to 45 minutes. Fortunately other players can bust you out of jail, or even bail you out.
The chances of succesfully busting someone are about a 75% chance, but getting caught in the act will put you in jail for twice the amount of time as the person you were busting (ie. your freind is in for 10 minutes, you get caught trying to bust them and you get sentanced to 20 minutes).