If any of the wh-family words (whom, who, which, whose) or that is used in the middle of a sentence to refrer to some already metioned noun, it is called a relative pronoun.
- The student, who came late, was punished by the Headmaster.
- The Old Man and the Sea which is a famous novel is my favourite.
- Nainital which is a famous hill station is visited by people in large numbers.
- Henry, whom I invited for dinner, is my neighbour.
- This is the boy whose parents are missing.
In the above examples, the higlighted wh-family words and that relate to some noun/noun phrases appearing before them. Therefore, they are relative pronouns.