An adverb phrase is simply a group of two or more words that function as an adverb in a sentence. Just as an adverb can modify a verb, adjective or another adverb, an adverb phrase of more than one word can further describe a verb, adverb, or adjective.
Adverb phrases typically answer the questions how, where, why or when something was done, as you'll see in the adverb phrase examples below.
Consider the following sentences:
I parked the car.
I parked the car here.
I parked the car right here.
I parked the car right here under the bridge.
The first sentence does not contain an adverb or adverb phrase at all. The second sentence contains the adverb "here" to describe where the car was parked. The third sentence contains the adverb phrase "right here," which emphasizes where the car was parked and employs a phrase instead of a single adverb.
The final sentence of the group contains a longer, more informative adverbial phrase. Note that "right here under the bridge" is a prepositional phrase that uses the preposition "under" and the object "bridge." In this case, the prepositional phrase functions as an adverb in the sentence. Since it modifies the verb to describe location, it is both a prepositional phrase and an adverbial phrase.