Navaratri means nine nights. That which is associated with darkness is night. What is this darkness? Ignorance is darkness. Man suffers from nine types of darkness. The purpose of the Navaratri celebration is to enable man to get rid of the nine types of darkness which have taken hold of him. When a reference is made to Devi, it signifies the unified form of Durga, Lakshmi, and Saraswati. The three together represent Shakti. Shakti is the energy that accounts for all the phenomena of Prakriti (Nature). Prakriti is energy and the controller of that energy is the Paramatma, the Lord. Prakriti is made up of the three qualities, Sattva, Rajas, and Tamas. As Prakriti is made up of these three qualities, to get control over Nature, man has been offering worship to Durga, Lakshmi, and Saraswati.  These are not goddesses but deified symbols of the three qualities. To acquire the grace of the Lord, man has to offer worship at the outset to Prakriti. – Divine Discourse, Sep 27, 1992