The durometer is a measure of hardness of a material and refers to the hardness of a wheel. The wheels are usually rated with a number, followed by an "A" to tell you how hard the wheels are. The lower the number, the softer the wheel. And the higher the number, the harder the wheel. Most freestyle wheels tend to be 98A and is a good duro for most surfaces. For rougher surfaces a softer wheel, like a 95A, will make your ride a lot smoother and you'll still be able to break free for shove-its and such. Softer wheels will also grip the ground better, but they also develop flatspots easier. Harder wheels will last you longer, but don't grip as much. There are pros and cons to both, so you just have to experiment to find the right wheel for you.