This page lists all snapshots associated with the virtual system, if any. To revert to a snapshot, click on it and then click the Revert button. To remove one or more snapshots, check the boxes next to them and then click the Delete button.
A new snapshot can be created using the form at the bottom of the page. If the percentage is set to anything less than 100, it is possible for the snapshot to completely fill up if there are too many changes to the virtual systems filesystem. Once this happens, it will be no longer possible to revert to the state in the snapshot.