A click reloads the content so that the site's content becomes accessible again. Unlike closing tabs, which removes the information from the tab bar, sleeping usually keeps the tab open in the tab bar but in an unloaded state. The feature is designed to save resources, memory in particular, by unloading tabs and putting them to sleep.
Tab snoozing, also known as tab sleeping or freezing, is not a new feature, but it is getting added to more and more web browsers natively.
