The CPU Optimizer feature optimizes your processor for the work you are currently doing on your computer, and can increase performance in the active application by up to 100%. By focusing the power of your processor on the application you are currently working with, CPU Optimizer can ensure that all of the required processing cycles needed for its performance are dedicated to the task at hand. Other applications, processes and services on your computer continue to run, of course, but the important program you are currently using is given priority for the processor.
The amount of Boost you select indicates the amount of preference the active application will be given on the processor. The higher you set this, the faster your active application will run, but at the expense of slower background programs (which you may never notice!).