please  explain  relative  pronoun ................

A relative pronoun is a pronoun used to mark a relative clause, and having the same referent as the element of the main clause (usually a noun or noun phrase) which the relative clause modifies.

An example is the English word that in the sentence "This is the house that Jack built." Here that Jack built is a relative clause modifying the noun house. The relative pronoun that marks the relative clause and refers (within the relative clause) to the house in the main clause. It can be considered to provide a link between the two sentences "This is a house" and "Jack built the house", where the house referred to in each case is the same.