Adjectives modify nouns. This means that they decorate nouns by adding some characteristic. The adjective big can make the noun dog much more interesting when it becomes a big dog. Rabid, scary, and ghost might be interesting too.
Adjectives can never modify verbs or any other part of speech. They are crazy about nouns and don't have time for any other part of speech.
In some languages, adjectives change their endings to agree with the noun they modify. Often adjectives will agree in gender and number with the noun they modify. For example, in French feminine nouns end with an 'e'. So if I'm an adjective that's decorating a feminine noun, I too must end with an 'e'.
French nouns are made plural by appending an 's'. So, if I'm a French adjective modifying a plural noun, I must end with an 's'.
Many languages exhibit this type of noun-adjective agreement.
exercise #1: What are the first five adjectives that come into your head when you hear the word "water"? What about "fire"?
exercise #2: Does English have the kinds of noun-adjective agreement mentioned above?