Problem: what is and where to put the fourth output (little black cable)?

I do not know where to put the fourth exit? and what is it used?
To be more clear, I plug the cable to my dreamcast, it remains the output video, the two audio outputs and there is a final output (small black cable) I do not know where to plug ... If someone one can help me ca make me happy because I do not know if it's because of AC, but suddenly my dreamcast does not display anything when I want to play, my TV stays black: s