That, which, who, whom and whose are considered relative pronouns when they are used to introduce a relative clause.

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

Can anyone tell me whose coat this is?

She turned off the news, which was depressing.

If a relative pronoun in a defining relative clause refers to the object of the main clause, it can be omitted:

John's the bloke (that/who) she used to be married to.

It was the squirrel (that/which) they had seen on the roof.

This is the dress (that/which) I bought yesterday.