Relative Pronouns are pronouns used to introduce adjective or noun clauses that are not interrogative. In the sentence, The man THAT I MENTIONED has come, the relative clause, that I mentioned, is an adjective clause modifying man. In the sentence, WHOM SHE MEANS, I do not know, the relative clause is, whom she means, and is a noun clause forming the object of the verb know.
The relative pronouns are who (whose, whom),which, that and what.
Who (with its possessive and objective forms, whose and whom) should be used when the antecedent denotes persons. When the antecedent denotes things or animals, which should be used.
That may be used with antecedents denoting persons, animals or things, and is the proper relative to use when the antecedent includes both persons and things. What, when used as a relative, seldom properly refers to persons. It always introduces a substantive clause, and is equivalent to that which. Ex: It is WHAT (that which) he wants?