Fishdr- that's the VMO, vastus medialis obliquus. It's on the medial side of the whole quad complex and acts to pull the kneecap so that it stays centered in the groove. In those of us of a feminine persuasion, it gets inhibited and weak so it needs extra love. It acts to help straighten the knee in the last 10 or 15 degrees which is why everyone is running around doing short arc extensions. There are better ways to strengthen the VMO. If the VMO is already weak the larger quads can substitute in a short arc, so you end up in the same muscle imbalance you started with.
(i'm thinking externally rotated straight leg raises with feedback from just resting your fingers on the VMO, and functional activities like step-downs in front of a mirror and posture work)
"If Americans want to live the American Dream, they should go to Denmark." - Richard Wilkinson