The meow of each cat can be very distinctive and different. Cats tend to use the meow in order to communicate with humans whereas they tend to use more body language, non verbal cues and vocalisation with other cats.
Cats will meow for a whole range of reasons. Usually it is because he wants your attention. Some cats will meow in the early morning to ask for food. They know that doing this gets them to response they want so they keep doing it! They also meow for playtime, attention or to be stroked or cuddled.
Certain medical conditions can lead to your cat to meow more often. Deaf cats will often meow louder than other cats. Some older cats with an overactive thyroid will start to vocalise more. Older cats with cognitive dysfunction may start to vocalise differently or more commonly. Female cats will meow a lot when in heat to attract and demonstrate this to male cats.
If there is a change in your cats meow, it sounds different, is more frequent or your cat is meowing while in the litter box it may be worth discussing these changes with your vet.