A word such as “quickly” that modifies a verb is an adverb.
Adverbs commonly describe how, when, where, how often, or to what degree an action happens. In “She ran quickly,” quickly modifies ran by identifying the manner of running. Other examples include yesterday, which can indicate time; here, which can indicate place; often, which can indicate frequency; and perhaps, which can express uncertainty.
The traditional school definition focuses on verbs, but adverbs can modify more than verbs. In “very bright,” very modifies the adjective bright. In “quite slowly,” quite modifies another adverb. Words such as certainly can modify an entire clause: “Certainly, the plan could work.” This wider function is why modern grammarians often describe adverbs as a flexible or “catch-all” category.
Many adverbs end in -ly, but that ending is not required. Fast, hard, late, and well can all be adverbs, as in “drive fast” or “work hard.” Conversely, not every -ly word is an adverb: friendly and lonely are usually adjectives. Adverbs are often confused with adjectives; adjectives modify nouns, as in “a quick runner,” while quickly modifies an action or clause.