It is just a chat messenger service, like Instant Messenger but it is used more world wide. So if you need to communicate with buyers around the world, they would see it in your ebay listings and and Skype you (chat).
They also offer phone numbers and I have one - that way all my business calls from the online business come in on my Skype line - on my computer and my home phone is not ringing off the hook or at some weird hour if it were international. Plus all calls worldwide are free using Skype.
Hope this helps clarify a little better if you were still in question.
-Laura
www.wholesaleoutletwarehouse.com