Stealing from Lindsay Ellis's explanation:

There are different street cat clans. The Musical follows the Jellical cat clan, which meets every year under the Jellical moon for their Jellical ball. At the end of the night, their leader, Deuteronomy, will choose one of them to "ascend to become a higher being."

Killed. He means killed. The entire musical is the cats volunteering as tribute for the yearly ritual sacrifice by bragging about how awesome they are.

That’s the best explanation ever thank you for relaying it to me XD

I don’t know what I was expecting from the musical but it wasn’t that.