--- It is the subordinate clause that is used to modify a noun or pronoun in the main clause. It can be introduced by the pronouns who, whose, whom, which, or that (and sometimes when or where).The pronouns used are called relative pronouns because they relate to a noun or pronoun in the sentence.
Is he the boy whom you met at the party last night? (whom relates to the boy)
This is a book that I like. (that relates to the book)