Show Me Your Crits is a common play on words in Massively Multiplayer Online Roleplaying Games. The original phrase would be Show Me Your Tits (See flashing) or Show Me Your Clits (See 3-way (The Good Kind)).
Crits are the internet slang for Critical Hits.
This can also be interpreted as a Taunt for other players to attack your player. You would taunt them by saying Show Me Your Crits or daring them to Crit you.