Relative clauses allow us to give further information about something in the same sentence.
For example instead of saying She is the girl. She goes to my school.
which doesn't sound very natural, we can say She is the girl who goes to the same school as me.
Defining relative clauses
When the information contained in the sentence is essential to the sentence, we use a defining relative clause and we don't use commas.
A pilot is a person who flies planes.
Water is something which we couldn't live without.
Non-defining relative clauses
When the information in the clause is additional and not essential to the meaning of the sentence, we use a non-defining relative clause and it is separated from the main clause by commas.
Steve and his wife, who is French, are coming to the party.
I bought my car, which is red, last month.
English Grammar Reference: Relative Pronouns