HTML stands for HyperText Markup Language. It's basically the programming language that tells your browser how to display things on your screen. If you aren't already familiar with this technique, it's probably not worth your time to learn it just to post on eBay. Use the preformatted screens that eBay provides or use one of the commercial auction posting applications would be my advice.

In my auctions, I use HTML to center, color, and size the text, but otherwise leave the things the way they are. I sometimes think the very busy and cluttered screens of the professional posters are a turnoff.