Your goal is to fill every space and reach the X. Drag the line to move it around, or use WASD/arrow keys to move your snake one tile at a time. You can't cross your own path.

If you need to undo a move, click on your snake's body and your snake's head will retract until it reaches that tile. Alternatively, you can use WASD or the arrow keys to backtrack. If you're really stuck, you can click the restart button in the top right corner or press R to restart the level.