The person on the video, known online as Tsoding (or by some as "mista azozin"), was writing a music visualizer program using the raylib library for writing videogames. raylib doesn't have code aimed at UI handling, meaning he had to manage the UI by himself. He likes doing a little bit of trolling, so that's why he picked that title.

Tsoding does by far the most entertaining recreational programming sessions I have ever seen on the Internet, so, despite them being quite long (about two hours), I recommend you watch at least a little bit of his videos/streams if you have time.

YouTube channel: https://www.youtube.com/channel/UCrqM0Ym_NbK1fqeQG2VIohg
Twitch channel: https://twitch.tv/tsoding

If you're interested specifically on this video in question:

The video: https://www.youtube.com/watch?v=SRgLA8X5N_4
The program's code: https://github.com/tsoding/musializer
The playlist containing the making of the program: https://www.youtube.com/playlist?list=PLpM-Dvs8t0Vak1rrE2NJn8XYEJ5M7-BqT