A manvantara, literally a Manu-interval, is the span over which one Manu presides as progenitor and lawgiver of humanity: 71 mahayugas, some 306 million human years. The office is the point. In Hindu cosmology the world's populations, its gods and even its great sages are not permanent fixtures but appointments that expire: each manvantara has its own Manu, its own Indra holding the kingship of the gods as a rotating title, its own set of seven rishis and its own divine cohort. When the interval ends, the roster changes. The scheme has a quietly radical consequence: Indra, the mightiest god of the Vedic hymns, becomes in Puranic time a job title held by a succession of beings, and the texts enjoy telling proud Indras exactly this.
Fourteen Manus reign in a single day of Brahma. The current Manu is Vaivasvata, son of the sun, seventh of the fourteen, which places the present roughly midway through the day; the names of the seven to come are already listed, a striking piece of prophetic bookkeeping. Each Manu also anchors law: the Manusmriti presents itself as the teaching of the first Manu, Svayambhuva, and every subsequent age receives dharma restated for its condition, so the manvantara joins cosmology to jurisprudence, one office governing both the calendar and the code.
Sources Vishnu PuranaAnonymous with Manusmriti (The Laws of Manu)Anonymous