Relative pronouns connect one phrase or clause to another one, creating what’s called a relative sentence. Relative pronouns can also add more information about the subject. They can be used as an appositive. Here’s a quick list: who, whoever, whom, whomever, which, whose, and that.
The person who made the mess should clean it up.
Michael, who owns the house, grows prize-winning roses.
The raccoon that bit the child is being tested for rabies.
The comma, which is commonly misused, is a necessary punctuation mark.