Also known as downclocking, underclock is to set your computer's CPU (central processing unit) to run at a slower rate than it was manufactured to run. Underclocking is done by users who don't need processor-heavy actions to be performed by their computers. The intention behind this setup is to put less strain on your computer's fan, reducing the ambient noise your computer outputs.
This procedure is the opposite of overclocking, which speeds up the CPU for faster performance, at the expense of your computer's hardware.
Setting up your hardware to run differently than intended can lead to unforeseen problems. Only attempt this if you are an advanced computer user.