An adverbial clause is a group of words that functions like an adverb in a sentence. It provides additional information about the action, such as when, where, why, or how something happens. Adverbial clauses usually begin with subordinating conjunctions like because, although, or when. These clauses can be essential for adding context to a sentence. For example, in the sentence "I will call you when I arrive," the adverbial clause "when I arrive" tells us the timing of the action. This helps clarify the relationship between different parts of the sentence.