When you use an acronym or abbreviation on a page you may well explain it but it is useful to have a reminder available when you use it again.
Create a new blank page and save it as abbr.html. Paste this in:
<p>The <abbr title="Worldwide Web">WWW</abbr> is amazing.</p>
Some browsers may add a visual clue that there is more information available but you should add it yourself using CSS to be sure. Some form of underlining, colour change or italics may be best.
One of the reasons for doing this is to tell a screen reader that this is an abbreviation so it can read it out differently.