After you have registered, you may set your signature. This is a piece of text that you would like to be inserted at the end of all your posts, a little like having headed note paper.
If the administrator has enabled signatures, then you will have the option to include your signature on any posts you make. If you have specified a signature, then the forum will automatically append your signature to any messages you post. You can disable signatures on a per-post basis by un-ticking the 'Show Signature' checkbox when you compose your message.
You may turn the signature on and off on posts you have already made by editing the post and altering the state of the 'Show Signature' option.
You can set your signature by editing your profile.