router-outlet directive

This is the documentation for Angular 6.
You can switch to the latest version Angular 7.

Acts as a placeholder that Angular dynamically fills based on the current router state.

<router-outlet name='left'></router-outlet>
<router-outlet name='right'></router-outlet>

A router outlet will emit an activate event any time a new component is being instantiated, and a deactivate event when it is being destroyed.