You can link several clips together so that when you drag them, they move in unison. This can be useful for video clips and their associated audio clips.
When a clip is linked, it will have an extra button on it to make it easy to unlink.
To link two clips together, select the clips and then click the "Edit" menu, followed by clicking "Link", followed by clicking "Link Selected Clips". The clips will now move together relative to each other's start.