A Protestant is a Christian that does not recognize the authority of the Catholic Church and instead follows the doctrines of the Protestant Reformation. The break between Protestants and Catholics is referred to as the Great Schism.
One of the defining characteristics of a Protestant is they believe that the Bible alone is all that is needed to have a relationship to Christ and God, and only though a personal relationship with Christ would ensure a soul's salvation. This stripped down version of Christianity made the rituals and policies established by the Catholic Church unnecessary, a distraction, and a temptation into corruption and greed in the eyes of the Protestants.
As a sidenote, in period, Protestants considered Catholics heretics and vice versa.