Relative pronouns relate two parts of a sentence together, usually by joining a relative clause to the rest of a sentence. Who, which, whose, that and whom are the five relative pronouns. 'The man who rang me a moment ago asked if I would be interested in a special offer.' - In this sentence, the word 'who' functions as a relative pronoun.
Try this English quiz to see how well you understand this aspect of grammar.
That is the factory in ...... my Dad used to work.
The woman who said hello was my old dance teacher. Although it can be used to refer to a person, 'that' is not correct as the subject of a clause (i.e. that said hello). You will hear 'that' used incorrectly in informal spoken English, however