Ehm, CPU is "central processing unit", a computer component. It does not have a light linked to its operation. If by CPU you mean the whole machine, call it "computer".
If that led is close to a symbol that resembles a lightbulb, then it's the power led, and tells if your computer is actually getting power or not. It should go on when you press the power switch and remain on until you shut down the computer.
If it is close to a cylinder symbol, it is the memory usage led, and tells if your computer is reading from its main internal memory device. It does blink randomly when in normal operation and if you don't experience other issues you can safely disregard it.
Other leds can indicate the wifi connection or whatever the maufacturer thought to place there, if it's not one of the above, try to describe it or find an image of it from google.
Please
follow this tutorial to fill your system specs (step 7 to do it automatically), so we can know what is the thing you have in your hands. Without knowing more, help provided will be limited.