Lord Shiva, the most revered God of Hinduism is considered the ultimate hermit. Looking at his reclusive nature demon Tarakasur asked for the boon that he could only be killed by the son of Lord Shiva thinking that Shiva is never going to have a consort. When Shiva married Sati he got terrified but Devi Sati soon killed herself in the yagna kund. This made Tarakasur extremely happy and he became fearless. He created havoc all over the world and his atrocities were increasing day by day. Looking at all this destruction all the deities (devatas) got worried and went to Devarshi Narad. Devarshi Narad told them that Devi Sati is reborn as Parvati to marry Lord Shiva.
He went to Devi Parvati and told her about her previous birth. On knowing about her previous birth she regained the insight of the eternal love between Mahadev(lord Shiva) and herself and she vowed to marry Shiva in this birth also. She asked Narad what she had to do to marry Mahadev. He suggested her to go for austerity.
Then Devi Parvati left everything and indulged herself in devotion of Mahadev doing rigorous penance. For years she left the royal food and had flowers, leaves, roots as food but could not please Mahadev. Then she only took leaves for food and slept on earth in the dense forest without any shelter but still could not please Shiva. Then she gave up food and water and completely indulged in meditation. This pleased Lord Brahma and he granted her boon that Mahadev will marry her and then Shiva marries her.
This form of Devi Parvati who wears white clothes and holds Japamala in one hand and kamandal in other came to be known as Ma Brahmacharini and is worshipped on second day of Navratri.