An adverbial phrase is a group of words used in the same way as an adverb to add further information or detail to a verb, adjective, another adverb, or even whole clauses.
The word 'phrase' is the key, as this means that it is more than one word, rather than a single adverb.
But let's look at it in more detail.
This can get quite confusing as you may hear these terms discussed in differing grammar books or websites in reference to adverbs:
They all have the same function i.e. to explain how, where, why or when something was done.
But the way that they tend to be distinguished in grammatical terms is that an adverb is a single word and an adverb phrase is two or more adverbs together.
However, an adverbial phrase is a more informative group of words that will contain other words apart from adverbs and may or may not actually contain an adverb.
So as you will see from the last one, units of words can function as adverbs without actually containing an adverb. In the last example, the adverbial phrase is actually a prepositional phrase acting as an adverbial phrase.
However, 'adverb phrase' and 'adverbial phrase' are often used interchangeably, so you should not dwell on which is which.
The key thing is that an adverb is one word, but an adverb/adverbial phrase is more than one.
Now we'll look at the most common types of adverbial phrase, which are:
An adverbial phrase of manner answers the question how?
These are all adverbials of manner, but there is only one actual adverb in there - very.
Adverbial phrases of time state when or how often something happens, or for how long.
Adverbial phrases of place answer the question where?
You will see then that in a lot of these examples, it is prepositional phrases acting as adverbial phrases.
This is when the phrase is placed at the start of a sentence.
The purpose of fronting is either simply to vary sentence structures to make a piece of writing more interesting or to draw attention to that part of the sentence because it is seen to be important.
Here are some fronted adverbial examples:
The difference here is quite simple.
The key difference between phrases and clauses is that a clause contains a subject-verb combination whereas a phrase does not.
As an example, take a look at these two adverbials of time (answering the question 'how long did she keep phoning for'), one a phrase and one a clause: