Backups = extra spots in the sign-ups roster used to get quick replacements for inactive players in a Mafia Game. Sometimes, players sign as backups if there are IRL constraints preventing them from joining the game when it starts, but still wish to play if a spot opens up later on. Usually backups are thrown in the game by hosts when a player wishes to be replaced or becomes inactive.
In rare cases, when no backups are available, a host may elect to redistribute the role of an inactive player to a ghost that was eliminated from the game early, but only when the ghost does not carry on extra information from the previous role (other than the publicly available information in the game thread). This is generally discouraged, as it may leak information on the role or faction of the player that is replaced.