Some examples of nouns functioning as adjectives would be the following:
The rock wall was difficult to climb. (notice the noun form “rock” becomes an adjective since it describes the noun “wall”)
The wood floor was very slippery. (notice the noun form “wood” becomes an adjective since it describes the noun “floor”)
His car engine was being worked on in the shop. (notice the noun form “car” becomes an adjective since it describes the noun “engine”)