This is a nice 3D game, a little like Space Invaders. You are riding the player, which appears as a circle on the screen like a gun sight. You can move the player left/right, up/down and, to a limited extent, forward and backwards.
Falling towards you are polygonal slabs. Shooting the slabs adds to your score.
If the slabs get past you and hit the back of the world, you lose a health point. In order to give the player a little more of a chance, whenever the player has lost a health point, the player is immune to damage for the next half second or so. This means that if two slabs get past you at once, the first one will damage your player's health by hitting the back wall, but you will be immune to damage when the second slab hits ? meaning that you'll only get one health point damage for the two closely-spaced slabs.
When you shoot a slab it releases a shower of coins that bounce along the floor. If you can maneuver your player to collide with a coin, it increases your health.
Try using View | Ride the Player to toggle off the default behavior of riding the player. This gives you a different view of the world.