When writing, we frequently introduce information that supplements or describes the things we're discussing. Sometimes that information is essential in that it defines the thing, place, person mentioned in the sentence; it is restrictive and such clauses (underlined) are not set off by commas:
The people sitting in the rear of the auditorium could not hear the speaker.
People who live in stone houses should throw glass.
On the other hand, some information merely adds to the discussion. You set off these parenthetical, non-restrictive comments (clause or phrase) with commas:
My mother, who was born in Hayward, California, was an expert homemaker.
The dark forest, staple of many a scary tale, was inhabited by fairies and brownies and wood nymphs.