A signature can be placed on the bottom of all your emails to include information such as your name, address, or other information that you want everyone you email to receive.  This saves you time from having to type the same information at the bottom of all your emails.  In Thunderbird, you can add a signature in just a few easy steps.  First, on the Toolbar, click on Tools and then "Account Settings":

On the Account Settings window, in the middle of the page you will see an option to put in your own signature:

Put in your signature, click "OK", and you are all done!  Now your emails will have the signature you specified at the bottom of each email.