Control the snake and make it eat all the apples to clear the level, while avoiding the walls. Simple, right? Think again! The snake constantly moves in a circle - pressing space (or clicking) makes it change direction. That is how you move, but wiggling and winding your way across the level.
As you eat you grow longer, which makes it even harder to navigate! You can slither off the edges and re-appear on the opposite side, but the very act of avoiding the walls can be a hair-pulling experience indeed.