The backdrop is what plays behind everything else, from the first second to the last. Open it with the Backdrop tab under the panel. video only
Background color
Background color sets the color behind everything. On its own, that color is your backdrop.
Clips and images
Click the + tile to add video clips or photos. The picker has Photos and Videos tabs, and Stock, Uploads and Recent underneath. Choose which part of a clip plays, then Add video.
Clips play in order, one after another, and you can drag one to move it. The dialog shows how many clips there are and how long they run. Click Update backdrop to apply it.
The image editor has a background color but no clips: it's the Backdrop row in the INSPECTOR.