Detents, sometimes referred to as click-stops, are positions on a dial (or a ring around a lens - typically the aperture ring) at which the control stops readily. This is usually achieved by a captive spring-loaded ball bearing, which settles into one of a series of shallow holes in the mating part.
Detents have several advantages. They allow the control to be set repeatably (that is, the position of f/8, say, is the same each time), avoiding small random errors in exposure. They may also prevent the control being accidentally moved after setting. Detents also allow the aperture ring (of a lens without automatic stop-down) to be set without taking the camera from the eye.
On the other hand, the presence of detents may make it hard to select values between them.