Heaviside Layer

The Heaviside Layer is referenced all throughout Cats. It's talked about by the cats as if it's a place, but as it does not appear in the show, it's unsure if it actually is, or is just treated as a place without being a physical place (such as "Heaven" just meaning death, but being treated as a specific place).

The purpose of the Jellicle Ball seems to be to invite Old Deuteronomy to where the Jellicle Cats meet, all to ask him to choose one cat to go to the Heaviside layer. In the end, Grizabella ends up being the one the audience sees sent to the Heaviside layer. This is where the musical ends, and so we don't find out exactly what happens to her, or what the Heaviside Layer is.

Celebration
The Heaviside Layer appears to be something that cats actively want to strive to go to.But either way, the cats of the show react positively to it, there is, after all, an entire ball dedicated to it, and Old Deuteronomy even visits them annually for it.

The Jellicle Ball is well known as one of the hardest and most demanding dances in musical theater, as it goes for 9 minutes and 30 seconds of nonstop dance. This could very well reflect how intense the ball is for the cats as well.

The Jellicle Ball is told to happen annually, when the "Jellicle Moon" shines bright. If the Jellicle moon is a special kind of moon, or just the moon for Jellicle cats is unknown.

Theories
There are many theories on what the Heaviside Layer is. None of these are confirmed true.
 * Being sent to the Heaviside Layer is basically being killed, like a ritual sacrifice
 * That the cats in the Junkyard are in something like Purgatory, and the Heaviside layer is real life.
 * When they say to be "reborn into a new Jellicle life", it's true, and the cat will be literally reborn/reincarnated
 * It is theorized that the cats who songs are about, are cats who are putting themselves forward and saying why they should go to the Heaviside layer.

Trivia

 * The Heaviside Layer is an actual layer in the Earth's atmosphere.
 * While the Heaviside Layer is not referenced in T.S. Eliot's Old Possum's Book of Practical Cats, he once wrote to a cohort an (albeit incomplete) suggestion where he proposed that eventually the cats were to go “Up up up past the Russell Hotel, up up up to the Heaviside Layer”.