Smothering is a combination of facesitting/queening and erotic asphyxiation, where the partner sits on the other's face to force oral sex or rimming. In doing so, oxygen to the brain is limited, which enhances sexual arousal. It is often associated with dominance and submission, and even humiliation. There are many parts of the smothering that can be sexually attractive: the lack of air, the weight of a body on top of you, the wetness, the smell, and the fact that you are also deprived of sight and so much use your taste and smell instead.
While many will have the person to be smothered simply lie on the bed so that they can mount them, special furniture and equipment can be purchased (or even made) to enhanced the experience. Special chairs with lowered legs and a hole in the seat allow the smotherer to be in the optimal position, where the skin around the vagina and anus moves to allow better access to the smothered.
There are also smotherboxes, where the smothered puts their head in and the lid is put on. The lid has a hole in it which allows the smothered access to the vagina and anus of the smotherer, who will sit on the box. The boxes can also be locked, which helps to enhanced the domination experience.
As this sexual activity involves cutting off air, it is best to make sure you completely trust your partner. They should know when you need to take a breath, or when to keep you hanging on just that bit longer to give you a bigger rush. If you are concerned, work out a system of communication - a safe word won't work as well with someone sitting on your face! Tapping their leg will help them know you need them to give you some more air, and if you are using a smotherbox, consider knocking on the box to let them know.