From SA Mafia Wiki
The Jailor, usually a pro-town role, chooses a player each night to imprison, both protecting and roleblocking the target simultaneously. The protected player is immune from nightkills (and in some cases all night actions), but may not perform any night actions if they have any. The jailor can thus use this role to either protect town power roles or to roleblock scum.
While uncommon, Mafia jailors have been seen in a few games. The role has proven to be great for Mafia strategy, as it can be played as a townie would use it while advancing the Mafia's goals at the same time. For example, if Surec claims Tracker, and tuckfard, a Mafia jailor, wants to keep him from tracking his buddies, he can claim he jailed Surec to protect him from being nightkilled, when in reality tuckfard only intended for him to be roleblocked.