Stereo is often referred to as the opposite of mono (monaural) sound; monophonic sound has a single channel centered within the sound field. In popular usage, stereo means simultaneous 2-channel recording or reproduction of sound with the aid of a pair of speakers.
Stereo sound, or in short stereo, is the reproduction of sound with the help of speakers connected to two or more independent sound channels by placing them in different directions and symmetrically in order to give a pleasant impression in accordance with the nature of human hearing.
These speakers can be as simple as a pair of headphones or as complex as a vast theater system or concert rig. The most modern equipment capable of producing sound is primarily designed to use stereo sound.
Our ear has evolved to locate sound sources in our environment. One ear or one sound channel cannot give enough information about the source of the sound to our brain. Two or more sound channels are necessary for us to better understand whether the source of a sound moving around us is far or near.
In order to experience this sense of reality virtually, for example, music or theater plays are recorded with multiple microphones and the sound recording is arranged accordingly. When we listen to this recorded sound through stereo headphones or a stereo sound system, our brain will interpret which direction and how far the source of the sound is.