What Comes Out of the Center Speaker In This Sound Format

I forget exactly what it's called but it's the format where it puts stereo coming out of all of your channels. (It's like "Stereo All", something like that) I've got a Denon 1913. Does a mono signal come out of the center or is it a left or right or what? I'm sure it's been explained a million times but let's talk about it here. Thanks for any help explaining this to me.