Peter the Hermit was a medieval preacher and leader of the People's Crusade, which was a popular crusade that preceded the First Crusade. He is known for his role in inspiring and leading a large group of peasants and lower-class individuals to join the crusade. Peter the Hermit's preaching and charisma were influential in mobilizing a significant number of people to take up the cause of reclaiming the Holy Land. His efforts contributed to the overall momentum and fervor surrounding the First Crusade.