A subordinate clause that describes a noun. Defining relative clauses specify who or what is referred to.

Examples: That's the man who was rude to my mother.

This is the dress which I bought yesterday.

John's the bloke that she used to be married to.

It was the squirrel they had seen on the roof.

Non-defining relative clauses are enclosed by commas and just give additional information.

Examples: The children, who were really enjoying themselves, didn't want to leave the party.

The question of Brexit, which no one seemed able to solve, was in the news again.

Non-defining relative clauses always require a relative pronoun.