For those who want text:
https://www.wikipedia.org/wiki/Ultraviolet_catastrophe
Summary: before the quantum nature of light was understood, a physical law (Rayleigh–Jeans law) was proposed to predict blackbody radiation at various temperatures. It worked pretty well in the infrared range, but miserably failed in the ultraviolet range. The failure of the Rayleigh-Jeans law helped give clues to get quantum physics started.
In 1900, Max Planck derived the correct form for the intensity spectral distribution function by making some assumptions that were strange for the time. In particular, Planck assumed that electromagnetic radiation can be emitted or absorbed only in discrete packets, called quanta, of energy
P.S.
I would like to encourage users who post videos - please, always try to include a text version or link to a transcript. For some of us, YouTube is not working (interferes with ad blockers and browser security features) and some of us aren't using it for various other reasons. Reminded of this again, I have included a recommendation about that on the side bar. :)