Your app could be used by specific people, that you can name. For example, you might sell your app to a Financial Services company, which has 20 people in their accounts department using the app. These are named users. They can change over time, and seats can be unassigned and assigned to a new named user.
Alternatively, your app could be used by anonymous, concurrent users. It might be an app in the iOS App store, for eg a conference schedule, used by large numbers of unknown people, at the same time. These numbers could fluctuate greatly from month to month. Note that we count the number of people using your app at the same time NOT the number of concurrent connections to your FileMaker Server.