Simple stores multiple copies of each page in its database for later retrieval. If a user should make a mistake when editing and not be able to reverse it (i.e. deleting a large block of text without having another copy available) the administrator can restore a previous version of the page, thereby recovering the data.
Keep in mind that when a page is deleted, its revisions will be deleted along with it. It is advisible that you still make regular backups of your database instead of relying solely upon revisions.
(Note: For administrators who would like to save space in their database, there's a setting for the maximum number of revisions to store for any given page. If you use this setting, you will only be able to retrieve that many revisions for a given page. For example, if you set the maximum to 25, you will only be able to retrieve the last 25 versions of any given page.)