Maslow's hierarchy of needs is a theory in psychology that argues that the human needs are organized in a hierarchy, having to be fulfilled on a crescent level, a lower level must be completely satisfied and fulfilled before moving onto a higher pursuit.
The five levels are Physiological needs; Safety Needs, Social belonging; Self-Esteem and Self-actualization.
Thus, Self-actualization is the highest level, being summarized in the sentence: "What a man can be, he must be."