Simple Definition:

An adverb is a type of word that is used to modify or describe a verb, adjective, or another adverb. Adverbs usually provide information about how, when, where, or to what extent an action is done. For example, in the sentence "She runs quickly," the adverb "quickly" describes how she runs.

Very Simple Definition:

An adverb is a word that tells more about how an action is done. It can describe how, when, where, or to what extent something happens. For example, in the sentence "He runs fast," the word "fast" is an adverb because it tells us how he runs.