An adverbial clause is a group of words with a subject and a verb that functions as an adverb in a sentence to modify the main clause (e.g., “until Leo comes back”).
An introductory adverbial clause (sometimes called a fronted adverbial) appears at the beginning of a sentence (e.g., “Until Leo comes back, we won’t be able to start cooking”). Introductory adverbial clauses are always followed by a comma.