An App is typically a client-side application created using the Interaction SDK which uses the platform's libraries to connect in real-time to the platform. Apps are all designed to respond to controls and triggers from the platform, allowing content producers to take full control of the application's settings and real-time content behaviour.
The importance of App Spec files
In conjunction with writing a new application, every App has its own App Spec.
An App Spec is a set of JSON documents describing properties of an App such as:
Supported Elements with Element level settings
Project level settings
Event level settings
To learn more about creating App Specs, read the App Specs guide.