Sadism means to get sexual satisfaction from tormenting, oppressing and dominating other people. Masochism means to get sexual pleasure out of being tormented and humiliated. Sadomasochism (also called BDSM) is when two people voluntarily mix sexual activity with one of them tormenting, binding, humiliating or hurting the other.
Accidents and injuries can take place during such sexual activities, so it is really important that it is voluntary and has safe limits. It is never right to pressure someone to take part in sex acts if they don’t want to, and this includes married couples.