noun \ˈma-sə-ˌki-zəm, ˈma-zə-also ˈmā-\
enjoyment of pain : pleasure that someone gets from being abused or hurt; especially : sexual enjoyment from being hurt or punished.

Simply put, masochists get off on being on the receiving end of pain/suffering. If you like getting spanked, you’re probably a bit of a masochist. Masochism is typically seen in D/s dynamics, but it is by no means limited to those dynamics. Masochists may have a partner inflict pain or they may inflict the pain themselves.  Approximately 30% of masochists are also a bit sadistic, which is the near opposite. Most sadomasochistic dynamics/relationships utlize a safeword to ensure safety in play.