It's just a little picture that goes underneath your name on your postings. You can upload one off of your PC onto the site by going to the "Edit Avatar" section from your control panel (labeled "UserCP" in the menu). For this particular forum, the picture can't be larger than 80 pixels wide and 80 pixels tall. Also, it can't be bigger than 19 kbs.
If you don't have any picture editing tools on your PC, you can google around for avatars- they have plenty of sites that offer them.