Thanks to the ever-increasing popularity of wireless speakers, single-speaker sound is making something of a comeback. Sure, Bluetooth speakers can play the left and right channels of a recording, but since they're just a few inches apart, stereo is a non-event.
Ideally, for desktop stereo the left and right speakers should be at least 24 inches apart when you're sitting a few feet away. With room hi-fi systems, the speakers should be at least five to six feet apart. With desktop and hi-fi systems, the listener must be equidistant from the left and right speakers to hear accurate stereo. If you're off center -- just a foot or so closer to one speaker than the other -- the stereo image will be distorted. Most of the sound will appear to come from the closer speaker. The stereo image that was so carefully crafted by the mix engineers will collapse.
The word "stereo" is a combining form borrowed from Greek, where it meant "solid," and it was used with reference to hardness, solidity, and three-dimensionality. When you hear stereo over a decent pair of speakers, the sound should take on a three-dimensional quality. With the better-sounding recordings, stereo doesn't merely string the instruments and vocalists in a flat line between the left and right speakers; there should be a sense of relative spatial depth between the players, and the stereo image might extend beyond the width of the left and right speakers.
Stereo is a fragile thing -- sound reflections off the floor, walls, ceiling, and furniture can confuse the stereo image. Furniture or other objects placed between the left and right speakers can be a problem, so some experimentation with speaker placement can make a big difference. Moving speakers out, away from the wall behind them can improve imaging specificity. Ideally, speakers should be near the seated height of the listeners' ears to produce the most accurate stereo imaging.
Stereo over headphones is a very different trip. First, with headphones you're always centered between the left and right channels -- that's great, but depending on the headphone the sound is mostly stuck inside your head. It's a rather unnatural perspective. There's not much depth or spatial dimensionality with headphones. Full-size open-back headphones are better. They're a bit more spacious sounding, so the stereo image is less crowded.
Stereo has been around as a consumer format since the 1950s, but a lot of listeners haven't really heard stereo. It's easy enough to give it a try.