An adverbial phrase (AdvP) is a linguistic term for a group of two or more words operating adverbially, when viewed in terms of their syntactic function.
Compare the following sentences:
In the first, soon is an adverb (as distinct from a noun or verb), and it is an adverbial (as distinct from a subject or object). Clearly, in the second sentence, in an hour has the same syntactic function, though it does not contain an adverb; therefore, a prepositional phrase consisting of a preposition and a noun (preceded by its article) can function as an adverbial and is called an adverbial phrase. In the third sentence, we see a whole clause functioning as an adverbial; it is termed an adverbial clause.