When you record an event with a mono mic, every sound it picks up is saved to a single channel. Since the sounds are all mixed together, when you play them back on your stereo, the sound coming from your left and right speakers is exactly the same. To make a stereo recording of the same event, you need a stereo mic or two mono mics. Stereo recordings are saved to two independent channels that aren't mixed together, so when you play them back, the sound at the left and right speakers is slightly different.
At a live concert, our brain knows where the instruments are located on stage because some of them are louder than others and their sounds reach our ears at different times. Even with our eyes closed, our brain can identify where every instrument is located. The directional cues that make this possible are described as imaging. Mono recordings lack imaging because everything is combined into a single channel. Stereo recordings are more lifelike because the sound is captured the way we hear it. By recording two independent channels with different levels and timing relationships, stereo sound provides the directional cues the brain needs to recreate the sound field of a live performance.
When you listen to a stereo recording, your position between the speakers of your stereo has a lot to do with what you hear. As you move around the room, some spots make it easier to pinpoint the location of instruments than others. Out of all these possibilities, there's one location between your speakers where music has the best imaging and realism. This sweet spot is another difference between stereo and mono sound. Stereo has a sweet spot, because the left and right speakers have different timing and signal levels. Unless you're properly positioned between them these parameters change from the actual recording. Mono has no sweet spot since the left and right speakers have the same timing and level. You can be anywhere in the room and hear the same thing.
The more signal information you have in a recording, the greater its bandwidth. Mono recordings have small file sizes because the process of mixing signals into a single channel doesn't capture that many sound frequencies. Stereo recordings have to capture a lot of frequencies to record the timing and level information in each channel, so their file size and bandwidth are higher. Since higher bandwidth puts greater demands on equipment, it's better to use mono for voice recordings where imaging isn't important.