A Busdriver (or Bus Driver) is a role who switches the night actions performed on two players each night. That is, all actions targeting Player A instead hit Player B, and vice versa. Generally, no one but the Bus Driver is aware that actions have been switched.
This can be very chaotic. For example, if the Cop investigates a Townie, but the Busdriver "switches" the Townie with a Mafioso, then the Cop will appear to get a scum result on the Townie. Or, conversely, if the Mafia decide to kill the Cop, but the Busdriver switches the Cop with another Mafia member, this would result in the Mafia nightkilling one of their own team members. And in that same example, if the Cop tried to investigate the switched Mafioso, the Cop would get a town result, since the Cop would be investigating themselves. And neither party would typically know what happened.
A Busdriver who can redirect where one player targets, as opposed to the other way around, and wins when the town loses, is called a Witch.