Spare CPU cycles are any computing power your computer "makes" but doesn't use. For instance, if you computer runs 24/7, but you are only using 1-2% of CPU Usage, the remaining 98-99% would be spare CPU cycles.
When gaming, surfing the web, creating a spreadsheet, etc... your CPU usage will obviously increase, thus reducing "spare cpu cycles", but it's unlikely that you will ever be using your CPU at 100% usage 100% of the time.
Hope that helps explain some... it's 2:30am and I just woke up with a headache, so it could be complete jibberish and someone else will better explain later.

Gamer - Thermaltake Element S | PC Power & Cooling Silencer 750 Black | Gigabyte GA-EP45-DS3L | Intel E8400 | Arctic Cooling Freezer 7 Pro | 4GB OCZ Reaper Ram | XFX 8800GTX | Creative X-Fi XtremeGamer | Seagate 7200.10 320GB
HTPC / Folder - Palit 9600GT 1GB Sonic | AMD Phenom 9600 | Corsair DHX 4GB | ECS GF8200A | OCZ StealthXStream 500
Thanks to Palit, AMD, Corsair and ECS for sponsoring the 2008 Folding Give-away!
