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, but does not contain an adverb; so a preposition, an article and a noun can function together as an adverbial. This is an adverbial phrase. In the third sentence, we see a whole clause functioning as an adverbial. It could also be called an adverbial phrase, but is more likely to be described as an adverbial clause.
Adverbial phrases sometimes consist of a pair of adverbs. Extra adverbs are called intensifiers:
(Try leaving out the phrase and see how it sounds).