In the sentence, "Phoebe's slightly larger red velvet cupcakes were a hit at the bake sale," the adverb "slightly" modifies the adjective "larger." An adverb is a word that modifies a verb, an adjective, or another adverb, and it often describes the manner, place, time, or degree of the action or description. In this case, the adverb "slightly" describes the degree to which the cupcakes were larger, indicating that they were only slightly larger than the other cupcakes at the bake sale. This helps to add more information and detail to the sentence, making it more precise and specific.