Stereo is two channels, left and right, the default format for music and most video soundtracks. By varying the level and timing of a sound between the two channels, a mix can place it anywhere on a line between the speakers — including a 'phantom center' that seems to come from a speaker that isn't there. Stereo carries a sense of width and space that mono cannot, at twice the data. Almost everything downmixes to stereo at some point, so engineers watch how surround and immersive mixes fold down, and codecs spend extra effort on stereo with joint-stereo and parametric tricks to save bits.
