Whether you know it or not, your iPhone and iPad actually tracks what you're doing in Safari for several reasons. The main ones being to speed up browsing and to store logins. On the down side, website data can also be used for tracking purposes. If you'd rather not have sites track you, clearing out this data now and again is a good idea.
That's it. All the website data currently stored on your iPhone or iPad is now gone. Depending on how much you browse, you may have noticed lots of data being stored. This is also a way to free up a little storage space now and again if you find yourself running dangerously low.