Every order status has the ability to trigger an email being sent to the customer. The process by which this happens relies on the status the order is being set to. For example, if an order goes from Pending to Shipped, the email template that is designated for the Shipped status will send to the customer.
To view and change these settings, navigate to Settings > Orders > Order Statuses.
Now whenever an order gets put into this status, and email is automatically generated and sent to the customer and/or admin alert email address. For more information on setting up email templates, feel free to take a look at our What are Email Templates and How do I use them? KB.