A relative pronoun is a type of pronoun that is used to introduce a relative clause, which provides additional information about a noun or pronoun in the main clause of a sentence. Relative pronouns include words such as "who," "whom," "whose," "which," and "that." These pronouns not only connect the relative clause to the main clause but also serve as placeholders for the noun or pronoun they are referring to.

Here are some examples of relative pronouns in sentences:

1. The man who is standing over there is my brother.

   (The relative pronoun "who" introduces the relative clause "who is standing over there" and refers to "the man.")

2. This is the book that I was talking about.

   (The relative pronoun "that" introduces the relative clause "that I was talking about" and refers to "the book.")

3. She is the student whose project won the competition.

   (The relative pronoun "whose" introduces the relative clause "whose project won the competition" and refers to "the student.")

Relative pronouns help provide additional information and clarify the relationships between different parts of a sentence. They are an essential component of complex sentences and are widely used in written and spoken English.