Explain the concept of a non-restrictive modifier.

A non-restrictive modifier is a type of modifier that provides additional information about a noun or pronoun in a sentence, but is not essential to the meaning of the sentence. It is often set off by commas or parentheses. Non-restrictive modifiers can be adjectives, adverbs, phrases, or clauses.

Unlike restrictive modifiers, which are essential to the meaning of a sentence and cannot be removed without changing the intended message, non-restrictive modifiers can be omitted without altering the core meaning of the sentence. They are used to provide extra details, descriptions, or explanations that are not crucial to the main idea.

For example:
- The book, which was written by a famous author, became a bestseller. (The non-restrictive modifier "which was written by a famous author" provides additional information about the book, but it can be removed without changing the main idea of the sentence.)
- John, a talented musician, played the piano at the concert. (The non-restrictive modifier "a talented musician" adds extra information about John, but it is not necessary for understanding the sentence.)

Non-restrictive modifiers are used to enhance the sentence by adding descriptive elements or providing additional context, but they can be removed without affecting the overall meaning or structure of the sentence.