CPU is the exact thing as "Processor", If I remember right it stands for Computer processing unit, or Central processing unit or something lul!
It's the brain in your computer that actually do all the calculations and where all the information should be sent. As ringo said, there's no harm if it use a lot of this.
It's the temperature that would cause harm if anything. But you shouldn't need to worry anything about that.
FH is CPU nomming application in general. It's usually what differs slow gameplay from fast in a game like this.
Most computers playing FH in a place like bonfire have a core at 100% load most of the time.
