A relative pronoun is a pronoun that marks a relative clause within a larger sentence. It is called a relative pronoun because it relates the relative (and hence subordinate) clause to the noun that it modifies. In English, the relative pronouns are: who, whom, whose, whoever, whosesoever, which, and, in some treatments, that. In addition, English has various fused relative pronouns, which combine in one word the antecedent and the relative pronoun: what, whatever, whatsoever, whoever, whosoever, whomever, whomsoever, whichever, and whichsoever,
A relative pronoun links two clauses into a single complex clause. It is similar in function to a subordinating conjunction. Unlike a conjunction, however, a relative pronoun stands in place of a noun. Compare:
(1) This is a house. Jack built this house.
(2) This is the house that Jack built.
Sentence consists of two clauses, a main clause (This is the house) and a relative clause (that Jack built). The word that is a relative pronoun in some analyses. Within the relative clause, the relative pronoun stands for the noun phrase it references in the main clause (its antecedent), and is one of the arguments of the verb in the relative clause. In the example, the argument is the house, the direct object of built.
Other arguments can be relativised using relative pronouns:
Subject: Hunter is the boy who kissed Jessica.
Indirect object: Hunter is the boy to whom Jessica gave a gift./Hunter is the boy who Jessica gave a gift to.
Adpositional complement: Jack built the house in which I now live. (similarly with prepositions and prepositional phrases in general, for example These are the walls in between which Jack ran.)
Possessor: Jack is the boy whose friend built my house.