"APP_INITIALIZER" is a built-in dependency injection token that allows you to execute custom functions during the application's initialization phase, essentially letting you run code before the app fully starts, enabling tasks like fetching configuration data, setting up authentication, or initializing essential services before the user interface is rendered.

Purpose:
To perform crucial setup tasks that need to be completed before the app becomes usable, ensuring data is ready when the user starts interacting with the application.
How it works:
You provide a function to the APP_INITIALIZER token, which Angular will call automatically during the bootstrapping process. This function can perform asynchronous operations like making API calls to retrieve configuration settings.
Common use cases:
Loading application configuration from a server
Checking user authentication before the app loads
Preloading essential data for the application
Setting up third-party services that require initialization