TT is an html tag that displays the included text in teletype font, that is, all the characters are of a uniform width, like on an old teletype machine. This is often used in html documents to show a quotation is being made, or that the author wishes to seperate what he is writing off from the rest of the document.
Without advocating style over substance, I would like to point out to people here on e2 that a little bit of formatting goes a long way, and that using the tt goes a long way.
Note: Since e2 does not use the code tag, I can't display the brackets around the tag, but you all know what they look like, right? And it gives me an excuse to demonstrate the tt font.