In grammar an adverbial is a word (an adverb) or a group of words (an adverbial phrase or an adverbial clause) that modifies or tells us something about the sentence or the verb. (The word adverbial itself is also used as an adjective, meaning "having the same function as an adverb".) Look at the examples below:

Danny speaks fluently. (telling more about the verb)

Lorna ate breakfast yesterday morning. (telling when the verb's action occurred)