Clicking this or pressing (Ctrl+M) will set the starting position to use when playing and/or editing multimedia objects.
Use Clip Start and Clip End to mark a range of frames for playback, for example, to play only a selection of the media sample. The default value is 0, the first frame. These settings can be used in both Playback mode and Edit mode. If you stop the play while playing a multimedia object, the current position will be set to the value of Clip Start.