If you're a cat lover, the sight of your cutie lying on her back is an endearing one. Although feline communication sometimes can be confusing, a kitty belly display usually is a pretty good thing. In fact, doing it often in your presence means your cat trusts you a lot.
Sometimes cats expose their tummies out of sheer relaxation. If your little guy is feeling serene and at peace with everything, he may show his belly. He may be feeling content, open and comfortable but at the same time still fully attentive and aware of everything going on around him.
If your kitty is feeling really content at the moment, exposing his belly may indicate the desire for a good-old-fashioned belly rubbing courtesy of you. Listen closely to your cat's vocalization. If he's purring as he lies on his back, it may be OK for you to touch his belly. However, depending on the individual feline, it may not be a smart idea at all. If you don't know the cat, don't do it. If it's your cat and you're not sure what he'll do, approach cautiously, casually. If your cat was simply reveling in the relaxation of it all and in no way wanted you to get tactile with him he may respond to your friendliness with a startling scratch or bite. Use context as much as possible when determining whether or not to attempt a rub. For example, widened pupils are generally not signs that a cat wants your touch at the moment.
Cats frequently show their tummies as a way of expressing unwavering trust. After all, felines feel and look pretty vulnerable to predators when their stomachs are exposed, so it's not a natural state for them to assume. If your cat freely and without reservations shows you his belly, it basically means that he's comfortable enough around you to exposed, at least in a defensive sense. Your fluff ball knows you're not going to attack him -- a pretty sincere compliment coming from a feline.
Although belly exposure is generally a positive thing, it can point to defensiveness in a cat, too. If a cat is in a frightening or otherwise threatening situation and feels he has no way out, he may assume a belly-up position as a technique for taking full advantage of all of his defensive tools, in particular retracted claws. When a cat is in this state of heightened protectiveness, he's certainly not feeling cuddly. Be on the lookout for growling and hissing, other hints of defensiveness.