a real world point of view
A standby props person is a member of the properties department who is on set at all times during the filming of a scene. They are responsible for the care and continuity supervision of the various props in a scene. They must respond to the requests of actors, directors and other personnel, and are responsible for explaining how individual props work to the actors and directors. They also tend to the onset maintenance of the props. If, for instance, the Doctor's sonic screwdriver should become unintentionally damaged during filming, they must immediately provide a replacement or find some way to quickly repair it on set. Because of the sheer number of props in any one scene, they almost always work in pairs to increase efficiency. They directly report to the property master, and are ultimately considered part of the art department.