This is a relatively modern move that is unmistakably intended to put a pawn on g4, a strategy that White normally begins with f3 or h3. But why not support this advance with the rook $2 Several of the best chess players in the world have attempted this ambitious idea, including Carlsen a few times previously.
1. e2e4  1... c7c5  2. g1f3  2... d7d6  3. d2d4  3... c5d4  4. f3d4  4... g8f6  5. b1c3  5... a7a6 