Angular comes bundled with an app_initializer token, allowing developers to execute some code before the application starts running. It provides a way to load data, perform authentication, or configure the application environment before the application is rendered in the browser.

The app_initializer token is defined as a provider in the @angular/core module. It takes a function as an argument and returns a Promise or an Observable. When the application starts, Angular invokes the app_initializer provider and waits for the Promise or Observable to resolve before rendering the application.

To use app_initializer in your application, you must define a function that performs the initialization tasks and returns a Promise or Observable. For example, you want to load some configuration data before the application starts. You can define an app_initializer provider in your app.module.ts file as follows:
import { NgModule, APP_INITIALIZER } from '@angular/core';
import { AppConfigService } from './app-config.service';

export function initialize(appConfig: AppConfigService) {
  return () => appConfig.load();
}

@NgModule({
  providers: [
    AppConfigService,
    {
      provide: APP_INITIALIZER,
      useFactory: initialize,
      deps: [AppConfigService],
      multi: true
    }
  ],
  ...
})
export class AppModule { }

In the above example, we define an AppConfigService that loads configuration data from a remote server. The load method returns a Promise that resolves when the data is loaded. We are also defining an initialize function that takes the AppConfigService as a dependency and returns a function that calls the load method.

The APP_INITIALIZER provider is then defined in the providers array of the AppModule. We pass the initialize function as the useFactory parameter and specify the AppConfigService as a dependency in the deps array. We also set the multi parameter to true to allow multiple initialization functions.

With the above setup, when the application starts, Angular will invoke the initialize function and call the load method of the AppConfigService. Angular will wait for the Promise returned by the load method to resolve before rendering the application.

In conclusion, the app_initializer feature in Angular is a powerful tool that allows developers to perform initialization tasks before the application starts running. It provides a clean and efficient way to initialize your Angular application. By defining an app_initializer provider, you can execute code to load data, perform authentication, or configure the application environment.