A relative clause forms part of a complex sentence. It is a type of subordinate clause that gives more information about a noun.
The dog that has three legs lives next door.
The underlined clause gives the reader important information about the dog.
Relative clauses usually begin with relative pronouns such as who, which, that and whose, although sometimes the pronoun can be left out.
The hotel that we stayed in was very comfortable.
The hotel we stayed in was very comfortable.
The relative clause always comes straight after the noun that is being described.