Hierarchy of Needs is a theory proposed by Abraham Maslow. It's a five-tier model of human needs, depicted in the form of a pyramid, with the essential need placed at the bottom. The needs described from the lower to the upper level are: physiological needs, safety needs, love and belonging needs, esteem needs, and self-actualization needs. The theory explains that the fulfillment of needs starting from the lower level leads to the desire to fulfill the next one.