'car' is a noun, because it gives a name to the object that we drive on the road.
I can describe my car as 'a small car', 'an old car', 'a red car'.
These words that describe my car are called adjectives.
So:
1. I have a white car. : 'white' is the adjective
2. I have a big white car. : there are two adjectives, 'big' and 'white'