A zone is a specified area, on a page, within a layout that you add content to. Each page can have an unlimited number of zones, and within each zone you can have unlimited number of content items, (known as playlist items) which in turn make a playlist.
When you first create a layout and you enter the layout builder you will see a zone already on your page with a text playlist item inside. You can either edit this zone, delete the zone to start from scratch or add more new zones.
Further Reading: Read here all other Zone Articles.