Documentation about different syntax in routes, Angular 4

Question

I'm using this theme: SB Admin Bootstrap 4 Angular 4. But I see a different syntax in routes:

App.routes

export const routes: Routes = [
  ...LoginRoutes,
  ...SignupRoutes
]

and in child routes SignupRoutes

export const SignupRoutes: Route[] = [
    {
      path: 'signup',
      component: SignupComponent
    }
];

I never seen this syntax: [...Routes] Is there some documentation/example