You're watching a cat get stepped on by accident, over and over, to one of the most elegant pieces of beginner piano ever written. The joke never gets old because the piece itself is so genuinely pretty, and the animation just lets it breathe. This is what happens when someone decides a children's song segment doesn't need to choose between being funny and being beautiful.
Neko Funjatta is a single music video made for the Minna no Uta song segment. Its source is the Flohwalzer, or Flea Waltz, a simple German piano tune that many beginners learn early on, since the fingering is easy and the result still sounds pleasant in harmony and rhythm. The tune goes by different names around the world, and in Japan people know it as Neko Funjatta, which translates to I Stepped on the Cat.