The difference between mono and stereo audio is that mono sound has a single audio channel, while stereo has two channels.
This means that if using mono sound, all connected speakers will play the same audio. Whereas, with stereo speakers, the sound is split between two audio channels, meaning that the left and right speakers can reproduce independent sound.
This adds depth and detail to the audio output and replicates natural hearing, with audio sources coming from different directions.
The majority of music and entertainment is recorded in stereo sound, so will produce this effect when played on stereo speakers.