The App Developer is a SDK for making apps for Flux OS. Its similar to Visual Studio. Yet, its not 100% working but you're able to make some simple applications for Flux. In the future it will be posible to also make apps for Flux OS in Visual Studio while running Flux OS.
At this moment, you're already able to code with the App Developer, however its not 100% complete. You can only code using vb.net yet. Maybe it will be posible in the future to also program in C#.
The app developer is nothing compared to Visual Studio, however it has the advantage that it automatically implements the functions of Flux OS. This can't be done that simple in Visual Studio and thats why its not posible to use VS yet.
Detailed documentation of how to program with the App Developer will be available in the near future.