When you start typing in a Web form with AutoComplete enabled, you will see a list of previous entries for the same field that begin with what you've typed so far.
You can select one to avoid retyping it. To see all stored entries for a field, click in that field and press the Down Arrow key.
To remove an entry that is erroneous, outdated, or private, highlight it using the Down Arrow (not the mouse) and press Del.