What is credit card test mode used for?

The test mode is used for setting up and configuring credit card processing at your site. When in test mode, you can verify your configuration is correct and that all your credit card transactions are flowing smoothly.

Warning: Make sure you de-select this option when you are ready to move into production mode. Most credit card transactions will appear to succeed whether running in test mode or live mode—but there will be no actual charges made if in test mode.