To put it simply, it is pre-recorded sounds created by a computer that was primarily used during the 80s and 90s when producing video games as this would save storage space which would be primarily used for essential files needed to run the game. While music still played a role back then, without the files that made the game what it was, the game wouldn't have been able to exist, so musical scores tended to be simple by design. However, even with these limitations, a lot of chiptune soundtracks sound incredible, and even today, we see plenty of developers embrace this method of sound design.