A frame measuring system that drops frames every few seconds so that the timecode keeps in time with a clock for video shot at 29.97 frames per second.

Notated HH;MM;SS;FF (example: 01;23;45;15 would be one hour, twenty-three minutes, fourty-five seconds, and 15 frames).

If you live in the US and are unsure, this is likely what your camera does.

see also Non-Drop Frame.