Sadism means getting pleasure — especially sexual pleasure — from hurting other people physically or psychologically.
If you've ever seen the word sadistic, you have a good clue to the meaning of sadism. People who are into sadism love to hurt other people — they enjoy it, especially in a sexual way. Sadism isn't always sexual, though. People in power, like supervisors, teachers, and even bus drivers, are often accused of sadism, because some of them seem to like abusing the people they have power over.