The patella is the kneecap, the small bone sitting over the front of the knee. Plural = 'patellae'.
The patella is an unusual bone, inasmuch as it is actually part of the tendon of the quadriceps muscle, the large muscle group that makes up the bulk of your 'lap'. A bone that is housed in a tendon is called a 'sesamoid' bone.
The fact that the patella is part of the quadriceps tendon means that it is involved in any action that involves the quadriceps, and a strong quadriceps is essential for the proper functioning of the patella, which acts as a fulcrum in standing, sitting or squatting.