How to switch on/off system load and running processes overlay
The setting you are looking for is, most likely, found in Settings &rarr; Development (might be called "Developer options"; I know on some ROMs this option is initially hidden -- but if it's hidden in your case, it would be funny how that CPU overlay was activated). Scroll down a bit there, and you should find the "Monitoring" section -- where you probably checked to "show CPU load".

 

As the screenshots show, the settings item is sometimes called "Development" and on other ROMs (here: Nexus) "Developer options". Simply check for something similar -- not sure if some manufacturer decided for a third term ;)