And for John: sorry for the satirical answers here

The level of detail radius is simply an option that defines how far from you the scenery should appear as detailled.
If you set this slider to the minimum, that means that you want the scenery to be detailled on a very short distance.
If you set it to the max, that means that you want to see all the possible details as far as possible.
This settings is there for the performance. Less details means better frame rates.