On day three of Navratri, we worship the Goddes Jagadamba (or Amba). In the Puranas (a series of 18 collections in Sanskrit of Hindu legends and religious instructions) Amba means mother and Jagath means Universe. Therefore, Jagadamba is the mother of the Universe.
It is believed before the start of all creation, the Goddess Jagadamba appeared in front of God Vishnu to commence the cycle of creation. Thus, it was through her that the universe was given birth. And she is the mother who has boundless love and forgiveness for her child. The name Jagdamba is used when Goddess Durga ( or Shakti) takes a peaceful form.
Because she is the peaceful mother avatar of Durga, devotees pray to her for well-being, prosperity, good health, family well-being, etc.