Manu is the guy who wrote Manusmruti, a "code of conduct" guide book for hindus.The stuff in that book ranges from reasonable to totally outrageous. It is the single most controversial hindu text and most critics of hinduism tend to use words and sentences from this book to make a point against hinduism.There is a theory getting popular among religion study groups that hinduism is a "word of mouth" religion, that it(texts) is passed down to future generations through series of social advises given to us by our dadi maa chacha chachi etc.https://en.wikipedia.org/wiki/Manusm%E1%B9%9Bti