rofl means Rolling On Floor Laughing which is used when you see something you think is funny but its really not and your friend starts saying you're retarded.
OH, MY GOD, I SAW THIS VIDEO ON YOUTUBE AND IT WAS SO FUNNY ROFL.
HAHAHHA MY BROTHER JUST FELL OFF HIS BIKE AND WENT TO THE HOSPITAL ROFL.
People think that it means "Rolling On The Floor Laughing" but in my mind it means "Rant Of Fucking Laughter" so if you want to ever say this make sure you know the real meaning