A relative clause is a kind of subordinate clause which is attached to a head noun within a noun phrase.

There are two kinds of relative clauses: restrictive or defining relative clauses and non-restrictive or non-defining relative clauses.

A restrictive relative clause is required to identify what is being referred to.

Read the sentence given below:

The girl who opened the door was very beautiful.
Here the relative clause is ‘who opened the door’, and it is attached to the head noun girl in the noun phrase ‘the girl who opened the door’. This relative clause is identifying because it is required for the identification of the girl under discussion. Without this relative clause, we would only have ‘The girl was beautiful’, which does not identify the girl.

A non-restrictive or non-defining relative clause is not required for identification. It merely provides additional information.

Consider the sentence given below:

Alice, who opened the door, was very beautiful.
Here the relative clause ‘who opened the door’ is not required to identify the girl who has already been identified as Alice.

In writing, non-restrictive relative clauses are separated from the rest of the sentence by commas.

More examples are given below:

The girl who answered the phone was very polite.
Here the relative clause ‘who answered the phone’ is restrictive because it is required for the identification of the girl under discussion.

John, who answered the phone, spoke politely.
Here the relative clause ‘who answered the phone’ is non-restrictive because it is not required for the identification of the speaker who has already been identified as John.

Compare:
The man who picked my pocket was arrested by the police. (Restrictive relative clause – who picked my pocket)
My brother, who is an engineer, lives abroad. (Non-restrictive relative clause – who is an engineer)