Adverbs help us to understand specifically how an action is performed. Someone might dance "quickly," or someone else might dance "sloppily" and someone else might dance "happily."
Adverbs are usually found right alongside the verbs they modify, but, unlike verbs (which are required for a sentence to make sense), a writer can choose whether or not any given action requires an adverb. You can say that someone dances poorly, but perhaps the quality of the individual's dancing doesn't really matter. What's most important is that someone is dancing at all.
Adverbs can also modify adjectives, usually when describing to what degree something is good, bad or ugly. For example, a dancer can be "frightfully" good or "awfully" bad or "terribly" ugly.
A rule of thumb: Many (though not all) adverbs end in "ly."