The application model is the information and services that is handled by the NetDaemon runtime related to the application instance. Instancing an app is the first step you do starting your automations in NetDaemon.
There are several way you can let the NetDaemon runtime instance applications. You can use a simple attribute
[NetDaemonApp] on any class and the NetDaemon runtime will instance it and run it or you can use yaml file to instance the application and provide configuration data.