The word Répétiteur is a French word which comes from the verb "répéter" meaning: "to repeat". The meaning here is "to rehearse", because when a musician rehearses (practises) he will go over the music many times until he has learned it really well. The German word is Korrepetitor.
A répétiteur will be a very skilled musician. He will be an excellent pianist who can play music on the piano which is normally played by an orchestra. He will often be able to help the singer to get to know the music by singing another part that is going on at the same time. He will be able to help the singer with technical problems with their voice, and also with pronunciation and musical interpretation.