What is an adverbial phrase?

I recently learnt 'to boot' , meaning in addition, as well. And someone was saying it is an adverbial phrase. I think I know what is an adverb, but never learnt of adverbial phrase.

I have seen some definitions but they seem vague and unhelpful:

• Adverbial phrases are a type of sentence or group of words that has the same ultimate meaning as an adverb.

Can somebody clarify the meaning of this grammatical term?