Manu is a title as much as a person. Hindu cosmology divides a kalpa, a day of Brahma, into fourteen manvantaras, and each is presided over by its own Manu with his own set of seers, gods and Indra. The first is Svayambhuva, born of the self existent, and the present age belongs to the seventh, Vaivasvata, son of the sun and the Manu of the flood. Seven more are to come. The scheme makes humanity's founder a recurring office rather than a unique origin, which is a different shape of time from a single creation followed by a single fall.
The name also carries a book. The Manusmriti, the Manava Dharmashastra, is ascribed to him and is the most cited of the Sanskrit legal treatises, setting out varna, the stages of life, the duties of kings, rules of marriage and inheritance, and penance. The ascription is traditional. Modern scholarship dates the text as it stands to roughly the last centuries BCE and the early centuries CE, and reads it as a composed work of a school rather than a revelation from the first man.
That distinction became politically live. On 25 December 1927, at Mahad in the Bombay Presidency, B. R. Ambedkar and his colleagues publicly burned a copy of the Manusmriti as a protest against the caste order it sanctions, and the date is still observed by parts of the Dalit movement. A figure who begins as the survivor of a flood therefore also stands, in modern Indian argument, for a text about hierarchy that he almost certainly had nothing to do with.