Mono (short for monophonic) means one channel of audio. Stereo means more than one. When you record a vocal into your DAW using one microphone and one channel on your interface, that is mono. Mono is one.
When we have a mono signal in our DAW, eventually it gets sent to a stereo bus. It’s technically still mono information, but that same mono signal is now sent both to L and R. This is what creates a centre image.
Mono information in a stereo environment equals centre information. As our ears hear the same thing from either side at the same volume and at the same time, our brains localise that as centre. This makes sense as that’s how it works in the real world. When a sound comes from in front of us, it reaches both ears at the same time and at the same intensity. That’s how we know it came from in front of us.
Stereo or stereophonic sound is the reproduction of sound using two or more independent audio channels
Couldn’t have said it better myself. In music, we use the left and right configuration of stereo as to create a three dimensional sound stage which would be impossible with just a single channel (mono). How would you make something sound like it’s coming from the left if you only had one speaker directly in front of you? You couldn’t.
From my blog
