As a kitten, a cat may meow because it needs to let its mother or owner know that it is hungry, and kittens also meow because they are cold or scared. For older cats, meowing is a form of communication.
Cats may also meow just to acknowledge their owner's arrival, such as their owner coming home from work. They can also meow when they want attention, when they are seeking food or when they are trying to get into a room where the door is shut. Owners of older cats may see an increase in their cats' meowing as well.