That would be the system doing nothing or simply idling..

When you open the Task manager and look under the Processes tab, you can see what amount of memory and CPU each process uses. At the bottom of the list is your System Idle Process which is basically the total CPU percentage left after all uses are totaled up for a quick view of your CPU use. Task Manager itself uses a little CPU so when you open it you don't necessarily see 100% maybe 99%. If while Task Manager is open you do something that will use CPU power (burn a disk or edit video etc.) you will see the process for that function show CPU use percentage and at the same time the total or System Idle Process will drop by the same amount.
When you here some one complain that some application or anti-virus is using 100% of the CPU, this is where they find that out since its process line will show 100% use and System Idle Process will drop to zero. When this happens though you often have to wait several minutes just for the Task Manager to open since it cannot get any CPU easily.