In the settings for payments, what is the meaning of "Test transactions in a live account"?
The language used varies by payment gateway, but they almost all include some sort of development environment that is completely separate from the production environment. Some will also permit test transactions in the production environment itself by letting you turn on test mode and use a test credit card number that passes the &quot;Luhn algorithm&quot; used to validate card numbers but is not a real number.
As this varies by payment gateway, you're going to have to consult the payment gateway's documentation. In Drupal Commerce itself, how you configure this differs between Commerce 1.x and Commerce 2.x. Make sure you're viewing module documentation that matches the version of Commerce you're running.