There are 3 ways to use it.
1.(popularized by the game warcraft) QQ which means Just quit your bad or just quit
2.QQ as in Quit Crying or your crying. Showing tears(although not the true meaning and orgin of it, that is how some people use it)
3. However in games especially warcraft it means, Stop Crying and Quit...
Quer - "Dude, your garbage I can't play with a idiot like you."
Doer - "QQ"
Quer - "Dude you suck, why don't you leave for us"