Pan, Mono, Stereo & Two Track Mono
To appreciate this it's best to wear headphones.
A MONO recording goes to one side only .. left or right. If this is coming from a left or right speaker then that is fine, as both ears hear the sound and will position the sound either to the left or to the right, depending on where the sound is coming from. But if one is wearing headphones only one ear hears the sound, and the other hears nothing. This is not a real life experience, and it'll do your head in.
If you COPY the sound, so you have exactly the same sound coming from left and right, then you will be able to listen to the music, but it will not have any depth, and sound as though it is coming from 'over the top of your head'. This is called TWO-TRACK MONO.
STEREO is when you 'pan' sounds to the left and the right, and the sounds going to each ear are different, both in makeup and in volume. You can see all this if you look at the waveform in Audacity.
I have come across the video below explaining all this with regard to playing a guitar. He manages to get a guitar to 'sound in stereo' by either having two mics on the guitar in different places; or recording what he plays a second time and trying to keep to the original as closely as possible. The second recording is not a COPY of the original, and as you can never play something exactly the same twice, if you put one recording to the left and the other to the right you will get a much better sounding guitar than you would get from just copying the original and having the same on both left and right (ie. ending up with two-track mono).
But another thing he looks at is just having the one recording, and putting a time delay on one of the channels. This gives a far better result for listening than having exactly the same coming to both ears.
If you are interested in how to make a good recording you will find this fascinating. Of course, we don't have to bother about any of this when playing/recording our keyboards as we have the ability to 'pan' sounds to left or right so our recordings are in Stereo anyway.
At 14:50 in the video he mentions another video he has put up, on John Lennon's "Imagine". This is a real eye-opener on chord harmony, and I have put up a Topic about this here:
Do a right-click to open this up in a New Tab.
http://www.tierce-de-picardie.co.uk/viewtopic.php?f=402&t=6526&p=58695#p58695