The word quietly is an adverb.
Adverbs are words that modify verbs, adjectives, or other adverbs. In the case of quietly, it specifically describes the manner in which an action is performed. For example, in the sentence ‘She spoke quietly,’ the adverb quietly modifies the verb spoke, indicating how the speaking is done. Adverbs often end in -ly, which makes it easy to identify them, although there are exceptions.