An email signature is that part of he email that appears after the main body. Mine looks like this:

--
Bert Jackson
Koa Interactive Media, LLC
Interactive Communications

23 H2 White’s Path
South Yarmouth, MA 02664
508 398 0026 P
508 398 0046 F
bert@koamedia.com
http://www.koamedia.com

Using an email signature makes it easier for people to contact you when needed. Often I am reading an email and need to call the sender. If their phone number is there, great. Otherwise, I have to hunt around. Same with the email address and web site. (But wait you say, isn't my email address already in the sender box? Well, yes. But if your email is forwarded to someone else, THEY will have your email address right there.)

Some people add a quote to their signature, some an image (I have mixed feelings about that).

The two little dashes ( -- ) above the email is a standard that says "signature below".

Most email programs and webmail programs allow you to set up a signature. Just look for "signature" in the help system.