From SA Mafia Wiki
Backups are roles that gain a power upon the death of the "standard" power role. For instance, a backup cop would become a regular cop if the "main" cop died. The two most common variations are Backup Cops and Backup Doctors.
Backups are considered powerful roles, since it means the mafia cannot just kill off an inconvenient player to remove the role from the game. To balance this, sometimes mods make backups weaker in some way to the standard role: e.g., a backup cop might be naive, or a backup doc a Weak Doctor.
A backup cop is often called a "Deputy", while a backup doc a "Nurse."
In rare circumstances, a player may be a backup and not be told as such; for instance, in a past game, the vanilla player with the highest votecount became a backup cop when the main cop died. However as stated, this is rare.