This is Nyan cat - it is an old internet meme that has retained a surprising amount of popularity over the years. It is an example of absurd humor. A moving picture of a cat running would be normal. Make it running through space and that would be weird. Replace its body with a pop-tart and it is weirder. Make it leaving a tail of rainbows behind it, and it is even weirder.
Actually, space + the rainbow makes the rainbow seem like some form of exhaust - like a rocket ship would leave behind. I don't know if that is weirder or not than the thought that it is just like a comet's tail.