The Game menu provides commands to enable the game mode and test newly created features.
Jumps to game mode so you can play the level within the editor (CTRL+G) (to exit game mode press ESC).
Enables the physics and AI within the editor (CTRL+P).
Makes the editing camera collide with the terrain so you can't fly under the terrain surface (Q).
Sets the player to the position where the current editing camera is.