ModuleId is a metadata property you can set when defining a component (in the @Component section). When using CommonJS, it allows you to use relative path inside the templateUrl and styleUrls metadata properties. For example, if your .ts files are in the same folder as your .html files, instead of having:

@Component({
  selector: 'my-component',
  templateUrl: 'app/my/my.component.ts'
})
export class MyComponent { }

You can have:

@Component({
  moduleId: module.id,
  selector: 'my-component',
  templateUrl: 'my.component.ts'
})
export class MyComponent { }

More information here: https://angular.io/docs/ts/latest/cookbook/component-relative-paths.html

If you try to use moduleId: module.id when using Webpack, you get this error when loading the bundled website:

Error: moduleId should be a string in "MyComponent". See https://goo.gl/wIDDiL for more information. If you're using Webpack you should inline the template and the styles, see https://goo.gl/X2J8zc.

With this loader, you can keep the moduleId: module.id and your code will work whether you are using Webpack or not.

This is handy if you are using something that does its own bundling (for example using karma-typescript for testing) and needs the moduleId to handle your relative paths.