An adverbial clause is a subordinate clause which takes the place of an adverb in a sentence.
Just like adverbs and adverbial phrases, adverbial clauses answer the questions where, when, how, to what extent, with what goal/result and under what conditions.
In addition, an adverbial clause may tell why.
Note how an adverb clause can replace an adverb and an adverbial phrase in the following Example:
Adverb: The Prime Minister gave a speech here.