By default, both the Start and End boxes are unticked. This means the video will play from the start of the video to the end of the video. When you tick the Start or End option, the Secs box to the right of the option will become active. Use the spin buttons to specify the start and/or end point for the video.

In the example above, the video will start 0.04 seconds into the video and will end after 45 seconds of playing.