Adjectives and adverbs add detail to the nouns and verbs that are the pillars of writing. There’s a big difference between a thin white sandwich and a greasy, overstuffed one. (See a discussion of using commas between adjectives.) When a little girl cries, is she crying softly, angrily, or hysterically? Again there’s a big difference.
Because nouns and verbs give writing its structure, try to build as much meaning into them as you can. For instance, if you say that the little girl whimpers, howls, or bawls, you don’t need to add any adverbs. (Learn more about limiting your use of modifiers.) Still, you’ll often need adjectives and adverbs. Just make sure they clarify rather than clutter your writing.