Page expiry allows you to set a date in the future when a page (or an entire section) are to be deleted from the site.
The expiry functionality is reached via the amend page screen. On this screen, you will see information about expiry - whether it is set, and if so, when for. Click on the "add/remove expiry" link in order to enter a wizard where you can set an expiry date.
Simply enter the date when the page should expire, and you click the button to return to the amend screen - which will now include information about expiry date. The page owner will receive an email warning them that expiry has been set.