Each time a page is published to the internet its state is saved and given a version number. This makes it possible to restore previous versions if you want to rollback any changes or you accidentally publish a page before it's ready and want to put back the previous version.
Step 1: Select "Versions" from the context menu
Right-click on the page name you want to see versions for and select "Versions" from the context menu.
Step 2: Select a version to restore
A new window will be displayed containing a list of all previous versions of the page. You can choose to sort the list by user and by date if necessary using the links in the right-hand column.
If you're not sure which version you want to restore simply select a row and then click on the "Show" button at the bottom. This will cause a new tab or window to appear containing the page as it appeared at that version.
Once you've found the page you want then select it from the list and click on the "Restore" button. This will cause it to be published and a new version created with the current date and time.