When you modify a certain piece of hardware for better performance, it is often referred to as "tweaking" it. Overclocking the computer's CPU or changing jumper settings on the motherboard are common examples of hardware tweaking. Removing system limitations and adding plug-ins or extensions to a computer's operating system are types of software tweaking.
Tweaking a computer is much like "tuning" a car (you know, the ones with the huge mufflers, big spoilers, and pimped out rims). It may increase performance, but is best left in the hands of the technically savvy. For example, overclocking your computer's processor may cause it to crash frequently, or worse yet, overheat and destroy the CPU. So, for most people, it is best to leave well enough alone.