First, you need to know what nouns are, because adjectives and nouns usually hang out together. I’m assuming you do know what nouns are and that you can find them in sentences. Then an adjective modifies a noun in some way by giving additional information about the noun. For instance, the noun “girl” can be modified by these adjectives: little, cute, tall, mean, crabby, etc. So I might write, “The cute, little girl sang a beautiful song.” In this sentence, “cute” and “little” are adjectives that modify “girl,” and “beautiful” is an adjective that modifies the noun “song.”