Your goal is to get as much sand as you can out of the box and into the pail. You can rotate the box by clicking and dragging with the mouse or by using the left and right arrow keys.
Navigate the sand through the box and out the opening so that it will drop into the red pail. If too much sand misses the pail, click "Reset" to try again
Once the pail is overflowing with sand, you can progress to the next level. You may also want to replay the level and try to beat your best time.
Source: The Game Website.