A muscle belly is, simply, the fleshy part of a muscle, and refers to the length of the muscle in question.
Probably the easiest muscle for an example is the biceps - someone whose biceps insertion is lower (closer to the elbow) will have a greater muscle belly, and his biceps will have a greater ability to grow (and give the arm a “fuller” look) than someone whose biceps attach higher (though they will have a harder time getting a high “peak”).
A rule of thumb I read somewhere (maybe this site) is that if you can stick two or more fingers between your biceps and your elbow, you have a poor insertion point for that muscle (in regards to bodybuilding).
Of course, I could be completely wrong, here - to be perfectly honest, it isn’t something I’ve ever really thought too much about. Add 50-60 lbs of muscle to anyone and they will look pretty damn impressive no matter how bad their muscle bellies may be.
In my opinion, it isn’t something to waste time thinking about. The majority of people who whine over it do so to justify a lack of progress and growth. You can’t change it, anyhow.