David
The Start orb is the circular button in the bottom left corner of the screen containing the Microsoft symbol on a blue background.
In the context in which it is written I would interpret clean as meaning remove.
You are removing any existing entries, which would otherwise be executed. These entries may conflict with other entries in the registry so the system needs to start again without the impediment of partly executed instructions. Changes to the registry are
made when files are not in use, namely when the computer is shut down and restarted. The need to do this arises because part of a transaction was completed when the computer was shut down but when the computer was restarted the transaction was not completed
as intended.
CAUTION
Modifying REGISTRY settings incorrectly can cause serious problems that may prevent your computer from booting properly. Microsoft cannot guarantee that any problems resulting from the configuring of REGISTRY settings can be solved. Modifications of these settings are at your own risk.