Constructivism is a learning theory where students actively construct their own understanding and knowledge through experiences and reflecting on those experiences. There are four key theorists who contributed to constructivism: Jean Piaget believed children learn through stages of cognitive development; Jerome Bruner thought learning relies on prior knowledge; Lev Vygotsky emphasized social learning and the zone of proximal development; and John Dewey advocated for student-centered learning. In constructivist classrooms, students are at the center of learning through hands-on activities and group work, with teachers acting as facilitators to help students build knowledge.