What is the third break light?

I'm having to register my corolla DX wagon, and they told me my third break light was out, I went to a Toyota service department and bought the third break light and put it in, but the light will still not work, I thought maybe it was the fuse but than figured that there would be other things connected with the fuse, but the only thing that isn't working is the light. So I was wondering what other possibilities there is for the break light not work other than the light bulb being out, cause I don't think that is it.