Top-grain leather is the grade of leather you'll find in "fine" leather goods and can be considered the middle-of-the-road quality of leather. It's used in the vast majority of purses for women, as well as small leather goods for men, like wallets that are sold by well-known designer brands.
It's made by splitting a piece of full-grain leather and sanding away any imperfections in the hide and stamping a fake grain on it. Usually, it's then treated and colored to provide a completely uniform look.
The finished product ends up being a bit plastic-like, and not nearly as durable as the best quality leather — full-grain. It will not age well with use, and will end up looking old and worn after a time.
This finish of leather is great, however, if you don't care as much about durability and more about the color of your Belt Leather Supplies, or its resistance to stain, as it has been heavily treated.