Jail, for those who are wondering, says “Opposing units played from hand go to jail instead of arriving. When a unit enters jail, any unit already there is released and arrives in play. (Jailed units aren’t in play. They’re discarded if Jail is destroyed.)”
So, assuming all your opponents play at least one unit on their turn, it would release the unit for the previous player who would then be able to ready said unit on their next turn. If I’m understanding that correctly, it makes the Jail pretty pointless in FFA matches.