Angular – What is the meanings of module.id in component?

In an Angular app, I have seen that @Component has property moduleId. What does it mean?

And when module.id is not defined anywhere, the app still works. How can it still work?

@Component({
  moduleId: module.id,
  selector: 'ng-app',
  templateUrl: 'app.component.html',
  styleUrls: ['app.component.css'],
  directives: [AppComponent]
});