Autocomplete is a feature where the site attempts to guess what the user is attempting to type. As they type in the box, if a similar term exists, they are presented these similar terms in a dropdown menu. Give it a shot in the box to the right. Try typing “a” and take a look at the resulting list. Then type “u” and see how the list now only contains one item.
This is a great way to help the user align what they are looking for with what is available on your site. You define the list of autocomplete terms and it can be as long as you like.
Also, this is an impressive feature as few website use it now. It works great for the search box.