You mean the text appearing after my handle in a post?
There's teh line below the window you type the post into that says 'Tagline: (optional, printed after your name on post)' and a box under it.
Enter text into there for your tagline.
Two of my favorite FReeper taglines.
"Ive got my eye out for Mullah Omar"
"Now with 20% more infidel"