This field lets you change the signature that is appended to all outgoing email messages that you send, including replies and forwards. You can only edit the default signature after you have purchased and activated your copy of TouchDown. You can enter some HTML code here for creating signatures with a variety of fonts, colors, styles, etc.
Here are some awesome sites to help get you create the code needed for an HTML signature:
If you are interested in adding a small logo or photo to your signature, please note that the image needs to be posted somewhere on the web (not your SD card as one would expect). Use that image URL to link your photo and you should be all set. The website linked above can help you manage that process as well.