By setting in/out points you can choose from which picture an input video starts and at which picture it ends. It also applies to the audio tracks. To change in/out points for a video do the following:

1. Move the preview slider to the first image you wish to keep.
2. Click Edit > In/Out Points > Set In Point
3. Move the preview slider to the last image you wish to keep
4. Click Edit > In/Out Points > Set Out Point.

Alternatively, you can select a file and click File > In/Out Points > Edit In/Out Points.