Adverbs modify verbs and contribute meaning of various sorts to sentences. The function of adverbs is to modify/describe verbs, adjectives, another adverb, situations and they may also modify a whole sentence. Ex: She wrote slowly. (Here, the adverb "slowly" is modifying the verb "wrote.") or, the other example
The landscape is impossibly green. (Here, the adverb "impossibly" is modifying the adjective green.