Shopify can store password-protected customer information including identity, order history, and current order status. Customers with accounts will have the pre-filled, making the checkout process faster.
To use customer accounts you need to have the Online Store sales channel.
How to Enable Customer Accounts
From Shopify admin, click on Settings
Click on Checkout.
Accounts are disabled - Customers will only be able to check out as guests.
Accounts are optional - Customers will be able to check out with a customer account or as a guest.
Accounts are required - Customers will only be able to check out if they have a customer account.
Click Save to apply the changes.