Internetz speak for the common radio communcation "Roger That!" "Roger" means "I have received all of the last transmission" in both military and civilian aviation radio communications.
rgr dat is commonly used by pc gamers and was popularized by its usage in the #1 online action game in the world Valve's Counter-Strike.
Randy #1: How'd that scrim go? Did you roll em?
Randy #2: rgr dat
Geek #1: Are you getting that new Android phone with the quad core processor?
Geek #2: rgr dat