Push and pull the little monsters to their destination, with the help of black and white fields. The white fields can push the monsters, and the black fields can pull the monsters. You can only move white fields when they're on a black background, and you can only move black fields when they're on a white background. So it's a matter of surrounding the monsters with the squares, in order to move them cleverly.
Click on the field that you want to move, and hold the left mouse button while dragging the field.