Post-rolls let you have a video play after the main video gets played. This can be a credits type page, or a brief commercial.
Each content item can have a post-roll label. This allows you to use the flexible label system to label all the content you want to show up as a post-roll of a particular video. If you have more than one content item linked up to the label, then the post-roll content is randomly selected.
Once you have properly labeled your post-roll content with a label of your choosing, you need to set the post-roll label property of the content you want to have the post-roll. You can do this in batch under the "bulk operations" on the browse content page, or on the "edit properties" page of the particular content item.