The tread surface on your tire is made up of many smaller surfaces known as “Tread Blocks”. The reason for so many surfaces is especially important when it comes to icy or wet road conditions. The Tread Blocks get their gripping power not from their many smooth surfaces but from the more numerous sharp surrounding edges. Siping improves the job started by your tire manufacturer by providing more of these gripping edges.
Siping is done by placing your tires (new or used) on a specially designed machine that rotates your tires while making small virtually invisible 90 degree cuts in your tread. Although the process is actually cutting your tread, it doesn't harm your tire in any way, it improves on it. Only under very close inspection can the sipes even be seen, and you're more likely to tell by your improved driving experience than by visual inspection.