The nonrestrictive clause is a dependent clause which is not essential to the meaning of the sentence in which it appears. The clause adds extra information but we can omit it without changing the basic meaning of the sentence. The Mauna Loa, which erupted last night, is a volcano in Hawaii. The band Nine Inch Nails, which had started to play again, filled the musical theater with sound. Do not punctuate restrictive clauses with commas.
Restrictive clauses are essential clauses.
Actors who do not know their lines cannot be expected to turn in a good performance. It is essential.