Your knee is the place where your leg bends.

If something or someone is on your knee or on your knees, they are resting or sitting on the upper part of your legs when you are sitting down.

The knee on a piece of clothing is the part that covers your knee.

If you are on your knees, your legs are bent and your knees are on the ground.

If you knee someone, you hit them using your knee.

If a country or organization is brought to its knees, it is almost completely destroyed by someone or something.