What does the durometer (a) means for a wheel?

Specifies the hardness of a wheel. A typical hard skateboard wheel is 100a and a typical soft longboard wheel is 78a. Softer wheels are more comfortable and are better for controlling your speed when sliding. Harder wheels are more forgiving when landing tricks and will be faster on smooth surfaces. 

The durometer scale can only go to 100 anything above it is just a marketing trick