The monkey stick is a toy that looks like a stick with a string and a ball attached to it. It works like this: first you hold the stick with one hand, then you swing the ball attached to the string around in circles with your other hand. As you do this, you can move the stick around and make the ball go in different directions.
It's sort of like playing catch with yourself - you swing the ball around and try to catch it with the stick, or you can toss the ball up into the air and catch it with the stick.
It might take some practice to get the hang of it, but once you do it can be a lot of fun!