Modifiers can take a sentence from zero to hero: they describe, qualify, or limit the meaning of another word.
Take a look at this puny little sentence; it's a total zero.
Brad broke his guitar.
Yawn. Go away, Brad. No one cares about you and your broken guitar.
But check out what a bunch of carefully placed modifiers can do:
At the end of the talent show, Brad, who only started taking music lessons last month, triumphantly broke his guitar against the drum kit, celebrating his killer, 14-minute solo with a rock star flourish, a sight that brought a tear to his proud mother's eyes and sent the girls in the front row into screaming hysterics.
Just like that, the sentence goes from Clark Kent to Superman. From Daenerys Targaryen to Mother of Dragons. From Peter Parker to Spiderman. From boring the readers to captivating them.
No, seriously: that's also a superpower. The pen is not only mightier than the sword; it's also mightier than a man of steel, a trio of flying lizards, or some truly suspect sticky finger-web.
And just like everybody's favorite blue mutant, Mystique (sorry, Beast), modifiers can take many forms: a modifier can be an adjective, an adverb, or even a phrase or clause that functions as an adjective or adverb. Yeah, it can get messy.
You could say that modifiers are defined by what the modify: Adverbs modify verbs, other adverbs, and adjectives. Adjectives modify nouns. So if it looks like a noun is modifying another noun, that first noun is probably an adjective. Because nouns aren't modifiers. Just sayin'.
Modifiers bring energy to your writing by providing additional details about what's being discussed. They make your descriptions more accurate and engaging, and they help hold the reader's attention.
Without modifiers, the average sentence is about as fun as watching paint dry.