What is an adverb clause?

An adverb clause (adverbial clause) is a clause that describes a verb, adjective or adverb, in the same way that a single word, compound, or phrase acts as an adverb. They are subordinate clauses.

Examples of adverb clauses:

The boy laughed when the teacher's wig fell off.

The bridge collapsed as the train rolled onto it.

He is not awake until he has his first cup of coffee.