"The seven great sages and before them the four other great sages and the Manus [progenitors of mankind] are born out of My mind, and all creatures in these planets descend from them."
The Lord is giving a genealogical synopsis of the universal population. Brahma is the original creature born out of the energy of the Supreme Lord known as Hiranyagarbha. And from Brahma all the seven great sages, and before them four other great sages, namely Sanaka, Sananda, Sanatana and Sanat-kumara, and the Manus, are manifest. All these twenty-five great sages are known as the patriarchs of the living entities all over the universe. There are innumerable universes and innumerable planets within each universe, and each planet is full of population of different varieties. All of them are born of these twenty-five patriarchs. Brahma underwent penance for one thousand years of the demigods before he realized by the grace of Krsna how to create. Then from Brahma Sanaka, Sananda, Sanatana and Sanat-kumara came out, then Rudra, and then the seven sages, and in this way all the brahmanas and ksatriyas are born out of the energy of the Supreme Personality of Godhead. Brahma is known as Pitamaha, the grandfather, and Krsna is known as the Prapitamaha, the father of the grandfather. That is stated in the Eleventh Chapter of the Bhagavad-gita (11.39)
Bhagavad-gita 10:6 Text and Purport
"After describing the Lila and guna-avataras, Lord Caitanya explains the manvantara-avataras to Sanatana Gosvami. He first states that there is no possibility of counting the manvantara-avataras. In one kalpa, or one day of Brahma, fourteen Manus are manifest. One day of Brahma is calculated at 4 billion 320 million years, and Brahma lives for one hundred years on this scale. Thus if fourteen Manus appear in one day of Brahma, there are 420 Manus during one month of Brahma, and during one year of Brahma there are 5,040 Manus. Since Brahma lives for one hundred of his years, it is calculated that there are 504,000 Manus manifest during the lifetime of one Brahma. Since there are innumerable universes, no one can imagine the totality of the manvantara incarnations. Because all the universes are produced simultaneously by the exhalation of the Maha-Visnu, no one can begin to calculate how many Manus are manifest at one time. Each Manu, however, is called by a different name. The first Manu is called Svayambhuva, and he is the son of Brahma. The second Manu, Svarocisa, is the son of the predominating deity of fire. The third Manu is Uttama, and he is the son of King Priyavrata. The fourth Manu, Tamasa, is the brother of Uttama. The fifth Manu, called Raivata, and the sixth Manu, Caksusa, are both brothers of Tamasa, but Caksusa is the son of Caksu. The seventh Manu is called Vaivasvata, and he is the son of the sun-god. The eighth Manu is called Savarni, and he is also a son of the sun-god, born of a wife named Chaya. The ninth Manu, Daksasavarni, is the son of Varuna. The tenth Manu, Brahmasavarni, is the son of Upasloka. Four other Manus are known as Rudrasavarni, Dharmasavarni, Devasavarni and Indrasavarni.
Teachings of Lord Caitanya, Chapter 8
The creator, Lord Brahma, is the incarnation of the passion mode of the Personality of Godhead, and creates the universal affairs with in every millennium by the force of the Lord's energy. The Supreme Lord Himself does the real creation by agitating His material energy, and then, by His order, Brahma, the first living creature in the universe, attempts to create the different planetary systems and their inhabitants, expanding the population through his progeny, like Manu and other progenitors of living entities
The Manus are sometimes incarnations of the Supreme Lord and sometimes empowered living entities. Formerly, many long years ago, Lord Krsna instructed the sun-god. The Manus are generally sons of the sun-god. The human race, or manusya-sara, descends from the Manus, sons and grandsons of the Prajapati, Brahma. The descendants of Manu reside in all the different planets and rule all the universe.
Bhaktivedanta Book Trust. HDG A.C. Bhaktivedanta Swami Srila Prabhupada.