A "signature" is text that is automatically added to the end of each of your posts.
Some people put a favorite quote in their signature, or their horses names, or the cliques they're a part of.
You can add a signature by going to your User CP and clicking on the "Edit signature" link in the menu down the left-hand side of the page. You can use BB code in your signature, but not HTML.
Please keep your signature to four lines or less, INCLUDING blank lines. (By this, we mean four lines on your actual posts. You may have to go back to your profile and make adjustments once you see how your signature looks on the BB.)
Long "sigs" are very tiresome to read over and over again. Please keep them short and sweet, and use your profile to provide more detailed information.