Monkey grip would be like wrapping your hand around the grip in circular fashion...bringing your thumb down to meet the middle finger of the strong hand. It's an example of what not to do...the idea being to have the thumb up high to allow the heel and meat below the thumb of the support hand to occupy the space behind the fingers, and below the thumb of the strong hand.
The strong hand applies pressure and support from the front of the grip (beneath the trigger guard) to the back strap. The support hand provides lateral stability (primarily) and also enhances the grip of the strong hand.