Knee push-ups are the easier push-ups you'll ever do. All you have to do in this exercise is to get in the original push-ups position but instead of having your feet on the floor, you put your knees which makes the weight of your body lighter and reduces the push-up angle which makes the push-ups easier but less effective too. This are the push-ups indicated for beginners or for people that are too heavy or have bones problems and can't lift too much weight. They are still effective but if you really want to get huge muscles don't add this exercise to your workout plan because you will be just wasting your time.
Details: Your knees are a very sensitive part of your body and in order not to injury them you'd better use a folded towel or a pillow under your knees.