Could someone explain to me what a CPU is? How could it harm my computer? Also, is having high memorie usage bad?

Your CPU is the actual processor that basically does the functions of a computer.   It's essential for any computer to run.  Having a high memory usage is different from CPU usage.  If you have high memory usage, it means too many programs are open, or some programs are using a lot of your memory.   High CPU usage usually means a program or program(s)/service or service(s) are currently running and require a lot of cpu power.

http://lmgtfy.com/?q=Can+having+high+CPU+usage+harm+your+computer%3F

The CPU is basically the brain of your computer. Your CPU won't harm your computer unless you take the cooler of it when it's on (even then the only part that will get damaged is your motherboard). Having high cpu usage isn't bad typically unless your not cooling it properly (you can download software to check your cpu's temps), typically speaking if you want it to last as long as possible keep it at 65 degrees or less (cores tend to be 4/5 degrees hotter than the cpu as a whole), 75 is acceptable for short periods of time but above that your hurting the cpu's life span.