Many people like to have a standard closing phrase, or details of their job title, address, contact details, and so on in their messages. A signature allows you to do this without having to type the information in each time.
If you choose to
[Automatically include my signature on messages I send],
every time you compose a new message you will see your signature text at the bottom
(you can then change or delete it for that message). Alternatively you can add the
signature to messages manually by clicking the
button at the top of any message: