Then the great king Bhagiratha having heard the same, went to the Kailasa hill, and betaking himself to a severe course of penances, at the expiration of a certain length of time obtained the favour of that worker of blessings (Siva). And, O protector of men! that same best of men, in order that his forefathers might have a place in heaven secured to them, received from that very Siva the fulfilment of his wish, namely the wish that the descending Ganga might be sustained.’”

In plain words: Bhagiratha went to Mount Kailasa and did strict penance again. After some time he won Shiva's favour, and Shiva agreed to bear the falling Ganga so the king's ancestors could reach heaven.