When is an adjective clause essential?

An adjective clause is a dependent clause that, just as an adjective, modifies a noun or a pronoun. It begins with words such as when, which, who, whom, whose, why, that, and where.

Adjective clauses can be either essential or nonessential. An essential adjective clause, also known as restrictive adjective clause, provides information that is necessary to identify the word it modifies. On the contrary, nonessential adjective clauses provide additional information about the word it modifies. Nonessential adjective clauses are set off with commas, while the essential ones are not.

harlem's  cotton club boasted the talents of duke ellington. singers such as bessie smith and billie holiday popularized blues and jazz vocals. jelly roll morton and louis armstrong drew huge audiences as white americans as well as african americans caught jazz fever.

what is this from, i cant tell what language he uses without the piece of literature