Face-sitting is a special position for oral sex.
The guy lies on his back and the girl kneels above his face, his mouth touching her vulva.
An advantage of this position is that the girl can easily make clear how hard or soft she wants to be stimulated. If she wants him to put more pressure on her lips or clitoris, she lowers herself. If he is playing too rough, she can move her pelvis up. She can position her body in such a way that he hits the hot spot.
Guys have very different opinions about face-sitting. Some love is, being surrounded by voluptuous flesh. Others find it suffocating, having difficulty breathing. One way to solve this, is for the guy to keep his hands between her vulva and his face. That way he can pull the lips apart and push the girl up when she comes too close.
This position is also used for anal stimulation, but then the girl faces his feet.