Relative clauses function as adjectives in a sentence. They are easy to recognize because they are introduced by the following relative pronouns.
Punctuation note: If the information contained in the relative clause is essential in limiting meaning, do not use commas. If the information contained in the clause is not essential use commas.
The book that she took with her was not the one she needed.
I lost the pictures that I like best.
The fortune (that) his mother left him is gone.
The book, which is one of my favorites, is missing.
Charles, who is my new neighbor, complains that my music is too loud.
Vegetables, whichever kind you eat, are good for you.