From SA Mafia Wiki
A Hider, also known as a Commuter, is a role which can choose to "hide" during the night. While hidden, the player is immune from all night actions, including nightkills and investigations. To avoid being too powerful, most mods put a limitation on the hiding by only allowing the player to hide every other night.
In another version of this role, the Hider must choose another player to hide behind, and if the person that the hider is hiding behind is killed, the hider will also die. This variant is fairly rare on SA.