Your company's locations and sets
This is a place to keep track of all the locations and sets that you might want to shoot at. As with the Directory, these locations and sets are meant to cover any number of projects -- potentially, all the places you might ever shoot at.
Here, a location is a general area or place in the real world, like a house, while a set is a place within that location where shooting would take place, like the house's kitchen, living room, or garage. A location (probably) has an address and can be found on a map; a set (probably) doesn't. When you create the location and its sets, you can include pictures, maps, medical information, and other similar things to help you make decisions about using that location in a project.
Because that's the point of setting up locations: besides providing you with a single place to keep track of all those places you have in the back of your head (a good thing in itself!), you can very easily connect one of these real world locations to a project location -- that lecture hall in Franz Hall (the set) at UCLA (the location) is going to be used for the lecture hall in Faber College (in your script).
These locations will then automatically show up in call sheets and other production-oriented parts of ProductionNext, and you'll be better able to make sure that you're not assigning more then one scene to the same location at the same time. You can do this in the project locations section of your project.