This action runs after all Caldera Forms files are loaded and the Caldera Forms autoloader has loaded and has registered Caldera Forms core classes. After this action you can safely use the Caldera Forms autoloader as well as any Caldera Forms classes or functions. If you are developing Caldera Forms add-ons, or are implementing custom code on your site for Caldera Forms, you should load that custom code in a callback function hooked to this action so that if Caldera Forms is disabled, your custom code will not run, instead of causing errors.
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.