What is a Jellicle Cat?

When Delibub, a red Somali cat from a rich family, is thrust out onto the streets, she has a fateful encounter with a grey tabby who takes her under his wing, introducing her to a tribe of cats, a new identity and a fantastical life she had no idea could even exist. But on the tribe's most important night of the year, a mysterious ginger tom threatens the tribe's way of life and her newfound family.