User Signature
A "signature" is an automatic ending to your email. Foe example, most people finish their messages the same way every time. They might always end with "Sincerely, Taro Yamamoto," for example. If you use a signature file, then you don't have to type this every time--it will be added automatically.
This feature is most useful when you want a large "signature," like the example below:

With a "signature" this big, you do not want to have to re-write it often. In order to set the "signature" file, click on "Change Signature" on the main page. You will see this:

Just type your signature here, then click "save." Next, you have a choice: if you wish to include the signature file in every email, then go to your Preferences and change the "Include Signature" preference to "Yes." Otherwise, you can choose to include your signature file on a case-by-case basis by clicking on the "Include Signature" box in each email that you compose.