An adjective is a word that tells us more about a noun. (By "noun" we include pronouns and noun phrases.)
An adjective "qualifies" or "modifies" a noun (a big dog).
Adjectives can be used before a noun (I like Chinese food) or after certain verbs (It is hard).
We can often use two or more adjectives together (a beautiful young French lady).
It is sometimes said that the adjective is the enemy of the noun. This is because, very often, if we use the precise noun we don't need an adjective. For example, instead of saying "a large, impressive house" (2 adjectives + 1 noun) we could simply say "a mansion" (1 noun).
Noun as Adjective