Missionary position is a way of doing sex with the man on top and the woman has her legs spread out. Many people like doing sex the missionary way though it can become boring through repetition, it's also called vanilla sex showing that it is pleasant but a bit ordinary. Unless done expertly it doesn't stimulate the lady's clit very well. Although the name of the position is widely believed to have come into existence either as a consequence of the actions or the teachings of Christian missionaries the truth is somewhat more complex.