Means Oh Really?
Often used with a white owl, occasionally decked out in different costumes for uses such as "Quite Rly" (With a monocle and pipe) Often used for sarcasm or just to be an internet nerd.
#1: The sky is blue.
#2: O RLY?!
#1: Old chap, the sky appears to be quite sapphire today.
#2: Quite rly.