What does prolific-usb-to-serial comm port mean?

That means that you're using a USB to Serial adapter like this:

http://sewelldirect.com/usbtoserial.asp

Prolific is one of the companies that makes USB to Serial chipsets, and when you are using one of their adapters it will show up on your computer like that. Many newer computer do not have serial ports on them so you need a USB to Serial adapter to be able to use serial devices with your computer. If you have a laptop and you're using a docking station then it might have a USB to Serial adapter inside of it which is using the Prolific chipset.