Matthew 8:25 And his disciples came to him, and awoke him, saying, Lord, save us: we perish.
26 And he saith unto them, Why are ye fearful, O ye of little faith? Then he arose, and rebuked the winds and the sea; and there was a great calm.

In English, a restrictive clause limits the word or phrase it modifies. For example, we might say, “I prefer swimming at a beach where there are no sharks.” The clause, “where there are no sharks,” limits the word, “beach.” Some beaches have sharks. We want one that does not. Restrictive clauses are not set apart by commas. They are considered essential to the meaning of the sentence.

A nonrestrictive clause, however, is set off by commas. This clause is set apart by commas to show that it is not crucial to the meaning of the sentence; it does NOT limit the word or phrase it modifies. Rather than restricting the word or phrase, it merely gives additional information. For example, we might say, “I prefer swimming in a chlorinated pool, where there are no sharks.” No chlorinated swimming pool has sharks. We are not limiting which ones of such pools we prefer. We are merely giving additional information about them; thus, the comma.