Deep throating is a form of fellatio, when all of an erect penis is placed inside a partner's mouth. The penis will enter the throat. The person whose penis is inserted is normally referred as the receiving partner, and the person who performs deep-throating is referred to as the giving partner.
It is difficult for most people to perform deep-throating, because of the natural gag reflex that happens when the soft palate is touched. Different people have different sensitivities to the reflex, but some people learn to suppress the reflex.
Usually, the giving partner's tongue is immobilized during deep-throating and they also are not able to suck.