WordReference Random House Unabridged Dictionary of American English defines 'relative pronoun' as follows:
What exactly do they mean a relative pronoun introduces a subordinate clause? That it has the function of a conjunction? Or even that it is a conjunction?one of the pronouns who, whom, which, what, their compounds with -ever or -soever, or that used as the subordinating word to introduce a subordinate clause, esp. such a pronoun referring to an antecedent.