Music visualization is the process of generating animated imagery based on a piece of music. The job of a visualizer is (usually) to perform this process in real-time for any given song. This topic has always been of great interest to me as I still remember being blown away by the Windows Media Player visualizer when I was 12.
There’s a huge amount of resources out there regarding visualization so this is really just the tip of the iceberg. I would encourage anyone interested in learning more to browse the previously mentioned Github list and find a developer who inspires you (my current favorite is Jordan Machado)!