A sig line is what you put after or below your nick/name at the end of an email or posted message. It can be any number of things but usually is a humorous or pithy quotation, or a link to one's webpage. It should be no more than one or two lines long.
The first line is a nick.
The second line is a 'sig line'.
Those are my principles, and if you don't like them... well, I have others. - Groucho Marx