It lets you route your incoming (source) A/V tracks onto the timeline. For example, let's say you want your source video to appear on V3 on and your source audio to appear on A5 and A6. you simple move the "V1" icon on the left(source) up to the "V3" icon on the right (timeline) and the "A1" and "A2" icons on the left to the "A5" and "A6" icons on the right, respectively.
Or, let's say you just want video from your source but no audio. Simply click on the source (left) "A1" and "A2" icons and they will separate from the icons on the right. This will make it so only "V1" will be edited into the timeline.
Lethal