This filter is used to change which Pods field is used for the label of a Caldera Forms field autopopulated with the results of an Easy Pods query.
Use Display Name For The Label Of Pods User Field
Use A Custom Field For Label Of Easy Pods Autopopulated Field
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.