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 the singer, they will switch sides.