Assets are the source video, audio, graphics, and subtitle files for your DVD. To become part of your DVD, source files must be added to the Assets container in the Project View and assigned to a track or menu.
1. Click the Assets tab to access the Assets container.
2. Drag asset files from the Finder to the Assets container.
Putting an item in the Assets container does not mean it will be included on your DVD. You must assign the asset to a track, slideshow, menu, or other item before it will be included.
Items in the Assets container are not actual files, but links to files (like Finder aliases). As a result, you need to keep the original files on your hard disk.
(This document contains information from Help Center, the help system included with your computer.)