It means that which is:
•totally ecstatically wondrous
•endlessly joyfully amazing
•full-on orgasmically blissful
Nathan Fillion instructing his ComiCon audience on what he has decided their participation will be with him at his panels: "The panel participation plan! If I say: DR, respond: DRATW. If I say: Full on, respond: Full on, DR. Be quick, be alert. Be DRATW." (DR is short for Double Rainbow)
"Why is the iPad so Dubul Rynbo?" (All The Way) (Double Rainbow, All The Way) DRATW
Firefly is so DRATW! Double Rainbow, All The Way!