It is a written confirmation of an order. This lets the customer know that their order is being processed. This refers to:

Why Order Confirmation is important?

Receiving a confirmation email promptly eases post-purchase anxiety, don’t leave customers hanging and with email automation, you can trigger an email to send immediately after purchase. Those are best Practices for order confirmation emails

Don’t forget to show your gratitude to the customer for choosing your brand. Customers have virtually endless choices when shopping online.