It is a funny joke of altering the layout of your gamertag on Modern Warfare 2 to look different. You and make it say something funny like "I R Pro" in blue or say "MLG is Gay" in pink and black. You can also include other charaters like X, A, B, Y, RB, LB, R, L, RS, LS, Dpad buttons in there.
"Dude your gamertag is so annoying with those LB and RB buttons. When you talk they flash on and off all over my screen."
"I really hate faggots who think they are so cool by using modded gamertags."
"Hey kid how'd you do that with your gamertag?"
"No shut up!"
20 random messages later...
"Please tell me! I'll give you Reecon."