People these days use "lol" but don't really "laugh out loud" and sometimes they don't laugh, at all, or even make a smirk. So, RL is now being used to differentiate from a boring "lol" and a real laugh.
RL can be used in multiple sequences back to back to show that you are truly laughing hysterically. Example: "RLRLRLRL"
Friend #1: "Hey bro, my dog totally just diarrhead all over my sisters lunch."