ROFL = Roll On Floor Laughing.
Which is a term widely used in internet chatrooms and instant messaging software.
Roffle evolved due to people readling ROFL like a normal word rather than spelling out each letter as R. O. F. L.
Roffle is therefore the casual friendly way to say ROFL and can be used as a verb in its own right:
"Ess Tee Eff Yoo"