A module is a class with the @NgModule decorator. To create a module we add @NgModule passing some parameters:

• bootstrap: The component that will be the root of your application. This configuration is only present on your root module
• declarations: Resources the module declares. When you add a new component you have to update the declarations (ng generate component does it automatically)
• exports: Resources the module exports that can be used in other modules
• imports: Resources the module uses from other modules (only module classes are accepted)
• providers: Resources that can be injected (di) in a component

A simple example:

import { AppComponent } from './app.component';
import { BrowserModule } from '@angular/platform-browser';
import { NgModule } from '@angular/core';
 
@NgModule({
  bootstrap: [AppComponent]
  declarations: [AppComponent],
  exports: [],
  imports: [BrowserModule],
  providers: [],
})
export class AppModule { }

Modules can be nested by using the imports parameter of @NgModule decorator.

We can create a core.module in our application that will contain generic things, like a ReservePipe (a pipe that reverse a string) and bundle those in this module:

import { CommonModule } from '@angular/common';
import { NgModule } from '@angular/core';
import { ReversePipe } from '../reverse.pipe';

@NgModule({
  imports: [
    CommonModule
  ],
  exports: [ReversePipe], // export things to be imported in another module
  declarations: [ReversePipe],
})
export class CoreModule { }

Then in the app.module:

import { CoreModule } from 'app/core/core.module';

@NgModule({
  declarations: [...], // ReversePipe is available without declaring here
                       // because CoreModule exports it
  imports: [
    CoreModule,        // import things from CoreModule
    ...
  ],
  providers: [],
  bootstrap: [AppComponent]
})
export class AppModule { }