Most people like to have a signature text and/or images, which lets them promote their own projects and interests.
If a mod or admin thinks your signature is inappropriate, they'll ask you to change it. They might change it themselves if your error is particularly large!
You can create and edit your forums signature in your User Control Panel, under the "forums" button:
(Note: Currently the sig editor says the max size for graphics is 550 x 150 pixels: this is INCORRECT! Please use the size restrictions at the top of this page.)
You can use the text editor to format text, add links, etc. If you want to add an image, you will need its URL - you can upload signature images into your Image Locker, and use that URL.