The goal of this game is to release the large 2x2 tile from the board through the opening centered on the bottom. To select a piece, click on it. Then it can be moved in any direction with the arrow keys, WASD, or HJKL. You can also click and drag with the mouse to move pieces. Only the large 2x2 tile can leave the board, so you must plan carefully to complete the puzzle in the fewest moves.
I also wrote a cleaner and more fully-featured version of this game in Java for CS 3733: Software Engineering, which is on my GitHub.