Do Action explained

do_action.php

<?php

// this can be placed anywhere in the plugin (before storing the form values in the database)
// $form_data could be an array of data which was created by the user
do_action('my_action_after_save', $form_data);

// we're telling wordpress to execute the code added in the second parameter method right where do_action has been specified
add_action('my_action_after_save', 'my_action_after_save_hook', 10);

/**
 * Use the form data and send it over to mailchimp
 * @hook my_action_after_save
 */
function my_action_after_save_hook($form_data){
	// write a code to collect the user submitted values and send it over to mailchimp
}