What it is
The spinal cord holds nerve cells that pass the order to move on to the muscles. In this disease they gradually die, the muscles stop receiving signals and become weaker and thinner, especially in the back legs.
The disease is inherited, so it can be noticed early, when a kitten should be climbing and leaping boldly but avoids it or tumbles.
It affects movement, not character: the cat stays just as curious and affectionate, only moving is harder. That is why any clumsiness in a young animal is worth showing to a vet rather than waiting for it to pass.
Signs you may notice
A kitten or young cat moves unsteadily, walks with short steps, finds jumping hard, and its back legs look weak and tremble.
When to see a vet
If a young cat moves unsteadily, avoids jumping or tumbles, book a visit so a vet can examine it. When getting a kitten, ask the breeder about the parents' health testing.
This is general information, not veterinary advice. For your own cat's health, tests and treatment, see a vet.