What it is
The large bowel draws water from the stools and moves them towards the exit with waves of muscle. When those muscles weaken, the bowel stretches, the stools linger and dry out further, and passing them becomes harder still. It is a vicious circle, and the stools held back in the bowel make the cat feel unwell, so it may lose its appetite.
The condition can develop after long-standing constipation, after a pelvic injury that narrows the passage, or from nerve damage. Sometimes no clear cause is found.
At home, the litter box is your best guide: notice how many times your cat passes stools and what they look like. Constipation that keeps coming back is not a small matter, even if the cat seems bright at first.
Signs you may notice
A cat spends a long time straining in the litter box, passes stools less and less, and the stools are small, hard and dry. It may vomit, eat less and turn listless.
When to see a vet
Book a visit if your cat has not passed stools for a few days or struggles to pass them. If it strains in the litter box and nothing comes out, a vet is needed immediately: this can also be a blocked urinary tract.
This is general information, not veterinary advice. For your own cat's health, tests and treatment, see a vet.