Celkem pěkně (jednoduchý základ) píše tenhle: http://ask-leo.com/what_is_the_system_idle_process _and_why_is_it_using_most_of_the_cpu.html

The fact is that most computers can never really do nothing. When the computer is on, the CPU's running and it must do something - even if that "something" is waiting for something real to do.

Think of it as your computer just twiddling its virtual thumbs, waiting for something more important to do. The computer's doing something (virtual thumb twiddling), but we wouldn't call that doing anything useful.

That's called being idle.

And the "System Idle Process" is the software that runs when the computer has absolutely nothing better to do.