This example shows off the Shadow Bias. This property exists primarily to help objects feel more grounded along a surface. There is a tradeoff, however, in that decreasing the value to make an object appear more grounded can sometimes lead to self-shadowing artifacts, as seen in the image below:
The diagonal streaks on the surface are the result of these artifacts, created from a low Shadow Bias value. It should be noted that you will generally not need to change this property, as the default will work in most instances.