ZP Atlas lets you use your own atlas database instead of GeoNames.org. ZP Atlas is an “add on” for the ZodiacPress WordPress plugin, therefore you need ZodiacPress to use this.
Use the ZP Atlas plugin if you don’t want to use GeoNames.org at all. Before you make this decision, please see Choosing an Atlas because it lists the benefits for each choice.
My personal advice for most websites (and especially small sites) is to simply use GeoNames, although I provide this ZP Atlas plugin for those who have the server resources to handle their own huge database full of more than 3.4 million cities.
See more ZodiacPress Extensions.