Stereo sound has 2 waves. One is for the left the other is for the right, which is why stereo compatible headphones have a right and left ear indicator. Below once again is a screenshot from the free audio recording app Audacity showing the left and right audio waves of a stern track.
Stereo sound tends to sound more realistic. For instance when listening to a stereo song using headphone your left ear might have instruments and the right will have the singer. Then after a stanza or a change in singer they will switch sides.
Stereo vs Mono its an under the cover feature most people won’t realize that this is happening in the background while you are listening to something. Although now you know why sometimes when you have one headphone in and your only hearing half the song. This is becoming increasing important because the iPhone 7 and up use stereo sound. If you have one of these iPhone listen to the top speaker then listen to the bottom now rotate it and notice tap the audio follows. This is want makes these iPhones sound so much louder. But it doesn’t stop there Apple as also added four stereo speakers to the iPad Pro.