Relative clauses have the function to specify an element of the main phrase. It can be the subject of the clause, an object or also the idea of the phrase.

Precision of the subject of the clause The cat that is on the roof belongs to my brother.   The car, that you see over there, is red.   Precision of the indirect object of a phrase   I didn't give him, who always offended me, nothing.   Precision of the direct object of a phrase   He bought the car that he liked so much.   Precision of an idea   The war broke out, which was a disaster.