An adverbial clause is a subordinate clause that does the work of an adverb in a sentence. An adverb is usually a word that modifies a verb, an adjective, or another adverb. An adverbial clause provides additional information about the action or state reported by the verb in the main clause. Examples:

John cleared the bush while James cultivated the land.
The students were making a noise when the teacher entered the class.
If you arrive late at the airport, you will miss your flight.
I couldn’t attend the seminar because I had a fever.
Although he is poor, he is honest.

For a more detailed audio-visual explanation of adverbial clauses with copious examples, watch the following video lecture:
https://www.youtube.com/watch?v=RAloM9CS5iw