This action runs before Caldera_Forms::process_submission() does anything. It is therefore an early entry point for submission for logging details early or preventing submission.
This action was added in 1.5.7 and exposes no parameters.
Where Does This Code Go?
When using WordPress hooks to customize Caldera Forms or other plugins you should not modify the plugin files, or you will lose your changes when you update the plugin. Instead you should create a small plugin to hold the custom code. It's easy, learn how here.
Technically you can add the custom code to your theme's functions.php, but then you will not be able to change your theme and keep these customizations.