Facesitting stories (sometimes also known as "face sitting", "queening", "face riding" or "smothering" stories) are tales where there is often an element of domination and submission involved.
The act of face sitting itself is where a person will squat over (not really "sitting" as such) their partner's face, lowering their genitals over their lover's mouth for oral sex. The dominating partner enjoys the feeling of empowerment as their sub is "forced" to perform oral sex on them - typically cunnilingus, but sometimes analingus too, as the mood fits!
Facesitting is popular in the kink / BDSM community as there are elements of domination, trust and control involved.