The Sitellite Change History is a feature available to some collections (depends on whether you set it them up with it or not) that automatically keeps track of every change made to each item in that collection. This allows you to see previous versions of an item, compare them to the current one, and restore previous versions if necessary, all through the GUI. It also allows you to retrieve deleted items.

The Change History is known as a versioning system. You can also think of it as a more sophisticated form of unlimited undo.

To see the change history of an item, either in the WebView or the ControlPanel, you would click on the little icon with the tiny "v1" in it. If you scroll over the icon it should say "Change History".