Quick answer: Cats meow primarily to communicate with humans. Excessive meowing usually means your cat wants something, but in older cats it can signal pain, cognitive decline, or hyperthyroidism.
Cats that have learned meowing gets results — food appears, you give attention, a door opens — will meow more frequently. Responding consistently to demands trains your cat that meowing works. Ignore the meowing and only respond when your cat is quiet to gradually reduce demand-based vocalization.
Cats over 10 that suddenly meow much more than usual, especially at night, may be experiencing cognitive dysfunction syndrome (feline dementia), hyperthyroidism, high blood pressure, or pain. New or worsening vocalization in senior cats always warrants a vet visit — don't assume it's behavioral.
Some breeds are naturally extremely vocal — Siamese and Oriental shorthairs communicate extensively through meowing as a normal personality trait. If you have one of these breeds, the frequency is likely normal regardless of how much they meow. Context and change matter more than absolute frequency.
Context and change matter most. Meowing that's always been present is likely learned behavior. Meowing that's new, worsening, or happening at night in a senior cat needs vet evaluation promptly.