Nava-ratri means "nine nights" and as the name says, it lasts nine nights and ten days. There are other Navaratris throughout the year, but the most sumptuous of all is the one happening during Autumn. It glorifies Nature (and the cosmic forces that move the entire existence) through rituals dedicated to the three main dimensions of Devi:
Durga, Lakshmi y Saraswati represent the three essential principles in Nature. Durga is the Divine Mother, Lakshmi the goddess of abundance and wellbeing and Saraswati is goddess of Wisdom, Music and Art. They are Love, Light and Wisdom. Respectively they correspond to Earth's inertia; the Sun's dynamic activity; and to the transcendental power of the Moon. First we all come from the Earth, then we are active in our everyday lives and, lastly, we have the capacity of awakening and fully refine as embodied Self. During Navaratri each of the three aspects of Devi is celebrated during three nights and the festival ends with a special ritual to Durga (the final highlight honors her one more time).
Here you will find a short video in which professor Douglas Brooks further explains the meaning of Navaratri.¬†https://youtu.be/gg4cR8SNs5w