Basically Skype is a program that runs on your computer that allows you to place phone calls to other people with Skype installed on their computer, for free. It's free computer to computer phone calls.
It will also do computer to a land line phone for a few cents per minute which is much cheaper than using a normal telephone company rate.
The reason it may be of interest to eBay sellers is that you can accept or make calls to anyone in the world for free. Which is important if you sell much outside your home country. It also allows you to accept calls on your computer and not give out your home phone number as someone above mentioned.
It also does video if you'd like. My wife will chat with my mom from California to Utah with our 3 month old on her lap so grandma can see and hear the baby.