I have enabled this session setting in my sandbox org. I did not see any difference interms of impacts to my system.which areas it will affect our system
What i need to test before moving it to the production.
You should test all of your VF pages, components, images, etc to make sure that everything is displaying correctly. Ideally, when you enable this feature, you should see no actual changes; its primary function is to make the session more secure in some situations (e.g. proxy servers that log URLs would log a GET session ID, but not a POST session ID). This feature can be toggled on or off, so I suggest simply turning it on and seeing if any of your users report unusual situations (images not loading, page errors, etc). You can always turn this off later if it causes problems.