An app instance can be started by playing on the play button:
You will then be redirected to the newly created instance. If you quit the instance before finishing, it will continue to work in the background. All your running instances are listed in the dashboard on the right:
You can either Connect to the instance to reconnect and see the instance again, or Abort the instance to stop it. On the top of the list, an Abort all button will stop all running instances after a confirmation dialog.