What it is
The gums surround the base of the teeth and guard it against bacteria. When they become inflamed they redden, swell and turn sore, and once the inflammation drags on, eating can hurt every time.
A cat cannot tell you about it, so the signs can be indirect: it lingers by the bowl but eats reluctantly, or picks softer food. Sometimes its mood changes too, and it turns irritable or avoids being stroked.
You can take a careful look at your cat's mouth at home by gently lifting its lip, but a proper examination belongs to a vet at each routine check-up.
Signs you may notice
A cat has bad breath, drools, eats carefully or avoids harder food, and rubs its face with its paws. The gums look red and swollen.
When to see a vet
Book a visit if your cat has strong breath, avoids eating, drools or rubs its face.
This is general information, not veterinary advice. For your own cat's health, tests and treatment, see a vet.