Portal zones are simply areas of the portal page, which is determined by the portal template that you choose. The simplest portal have one zone, which covers the entire portal page. Some portal templates have more than one zone. The purpose of the zones is to break the portal page up, to better organize all the links. Kind of like a tiled layout, if you will. If you select a template with more than one zone, you will clearly see this in the portal item list. When adding new items, you can select which zone to put them in.