Stereo supplies a sense of separation and depth; whereas mono does not, it has a more singular sound. Both terms are often used when referring to amplifier connection, but there are significant differences between the two.
Stereo has a left and right signal that are similar, but not exactly the same. These two channels give texture and color to the audio. If one instrument or voice is only produced in the left channel, it will seem to originate from the left side of the listening area. If a particular sound is only slightly louder in one of the channels, that sound will seem to originate off center slightly toward the channel in which the sound is louder.