Co-op mode

Co-op mode is simply a mode where you can get a friend to help you with 1-Player modes over Wifi or directly.

Over Wifi Connection
Over Wifi Connection you can get friends to help you with the Home-run Contest game mode and the Multi-Man Brawl game mode.

At the House
You can get a friend to help you with the Subspace Emissary, Home-run Contest, or Multi-Man Brawl