Guard Duty is a repeating mission that allows a survivor to add a defensive bonus to the building they are assigned to and a lesser bonus to any buildings adjacent or next to the occupied space. The strength of this bonus is linked to the survivor's Defense skill. When zombies massed outside the fort attack a building, that building's defense rating is what stands between you and destruction. If the building's defense (including guard duty and other bonuses) is lower than the number of zombies, they might get through, causing you to lose territory or survivors!
Survivors can be assigned to Guard Duty on any building inside your fort. Additionally, when a survivor finishes a mission, they'll automatically return to the closest square in the fort and go on Guard Duty until re-assigned to a new mission.