Its the percentage being used by your computer Lona,ie your ram,virtual memory and cpu (I think).
Each time you run a program it uses a bit more of them and the longer you are surfing and the more windows you have open the more it will use.
Things like Incredimail and Cd Burning Progs and P2P stuff have a dramatic effect on my computers resources so if I am using any of those I shut as much as I can down on the rest of my pc.
Occassionally you may find it helpful to reboot your computer as even when you think you have closed some programs they are still running in the background unless you have exited fully from them