A restrictive clause contains essential information and should not be offset by commas.
Restrictive clauses provide critical information about the noun they modify, and their removal would change the meaning of the sentence. For example:
- The book that is on the table is mine. (The clause "that is on the table" is essential to identify which book is being referred to.)
In contrast, a nonrestrictive clause provides additional, nonessential information and is usually set off by commas.