Question

What exactly is stereo sound? What is the difference between mono and stereo?

Answer 1

Stereo is a binaural (two channel) representation of a sound source (instrument or voice) or collection of sounds (mix).  True stereo requires two inputs which composite the stereo image; although a mono signal can be given some special information via different types of processing.

A mono sound source alone cannot be converted to stereo by copying it, splitting it, or panning it center.  This simply results in ‘dual mono’, which is not stereo and contains no stereo information.