A Tracker is an investigative role which can follow another player and see either who they targeted that night, if anyone. Trackers are extremely useful for discovering power roles, as vanilla players do not have night actions.
Scum Trackers are used to determine which townies have power roles so they have a better idea of who to either nightkill or attempt to get executed. Scum Trackers also have the advantage of being able to roleclaim their actual role without being automatically considered as having a scum role.
Roles that can perform night actions without being tracked are known as Ninjas, or simply "tracker-proof."