Well, basically, the CPU usage is how much of your CPU your are utilizing at that time. Thats not very important, as most programs will not utilize much of it. Then again, mine reads 99% at all times because I run Folding@home.
The memory one is a bit more important, where it says "available" is what you want to look for. if that drops real low, then you start having problems. Nce thing is, under the process tab, you can see how much ram every program is using.