A relative pronoun plays the role of a noun phrase within that clause.
In most cases the antecedent is a nominal (noun or noun phrase), though the pronoun can also refer to a whole proposition, as in "The train was late, which annoyed me greatly", where the antecedent of the relative pronoun which is the clause "The train was late" (the thing that annoyed me was the fact of the train's being late).
Other arguments can be relativised using relative pronouns: In some languages with gender, number, and noun declensions—such as German, Serbo-Croatian, Hindi, and Latin—the relative pronoun agrees with its antecedent in gender and number, while its case indicates its relationship with the verb in the relative or main clause.
This suggests that relative pronouns might be a fairly late development in many languages.
[4] In English, different pronouns are sometimes used if the antecedent is a human being, as opposed to a non-human or an inanimate object (as in who vs. that).