Cats can meow for many reasons, some being rather obvious and others not as much. All cat owners will have experienced a hungry cat meowing for food. It is very obvious what your cat wants from you when it is meowing and pacing around the kitchen (or sometimes right in front of where you keep your cat food!). Your cat may also meow when you come home, which probably means your cat is just glad to see you and would like some affection.
Female cats in heat will meow constantly as part of their breeding instinct. They do so in order to advertise to male cats their readiness to mate, and this can often occur for prolonged periods at all times of the day (and night!).
Cats can also make weird noises when hunting, especially when their prey is unreachable, such as a bird sitting high up on an opposing roof. The specific noise your cat makes can vary, but is often described as "chattering" or "bleating". It is unclear exactly why they do this... some theorise that it is a sound of frustration or anticipation; others think it is a deliberate attempt to tempt the prey into investigating the strange noise.
The key to understanding your cat's meow is to realise that meows are only a part of their attempts to communicate. You should look for other cues such as body language, context and environmental cues. The same applies for any other noises your cat may make, such as hissing, growling or of course a delightful purr.