Ahhhhh.... it feels so good to have an effect named after yourself.....
But seriously, the Magnus effect, or force, is simply put that a spinning object (like a tennis-ball or golf-ball) will create a "whirlpool" of rotating air around itself. On the side where the relative wind and the "whirlpool" of air is flowing in the same direction, the velocity of the air will increase, the pressure will drop, and we have lift. The opposite will happen on the side where the relative wind and the "whirlpool" of air is flowing in opposite directions.