Answer:
Masochism means deriving sexual pleasure from receiving (some kinds of) pain - commonly, whippings and other beatings, and humiliation.
(The term was coined in 1886 by Richard Krafft-Ebing, who named it after Leopold von Sacher-Masoch, without asking the latter for permission. Sacher-Masoch had written a novel entitled Venus in Furs, which was published in 1870. In the novel a very beautiful woman in furs and shiny high boots whips her man, treats him as her 'slave' and humiliates him. In the novel, they both do this by agreement and both enjoy themselves).
In a much looser sense, masochism is sometimes used in connection with sexually submissive people.
The word and its derivatives are also used in a figurative sense as in 'You must be a masochist to do that course; it is such a difficult course'.
In the 1920s some psychologists - mistakenly, I believe - claimed that every masochist is also a sadist and every sadist a masochist. (A sadist gains sexual pleasure from inflicting pain on others). In other words, they claimed that there are only 'sado-masochists, and this view was integrated in the 1940s into dubious theories about 'the authoritarian personality' and used in attempts to explain Fascism.
It is now widely held that masochists are usually sexually rather playful, while 'real' (rather than purely playful) sadism shades off into psycopathy.
An important insight gained is that much sexual activity also involves the exercise of power - and the submission to power.