Localizes a registered script with data for a JavaScript variable. https://codex.wordpress.org/Function_Reference/wp_localize_script

functions.php

// Register the script
wp_register_script( 'custom-scripts', get_stylesheet_directory_uri() . '/js/custom-scripts.js' );

// Localize the script with new data
$translation_array = array(
    'some_string' => __( 'Some string to translate', 'plugin-domain' ),
    'a_value' => '10'
);
wp_localize_script( 'custom-scripts', 'object_name', $translation_array );

// Enqueued script with localized data.
wp_enqueue_script( 'custom-scripts' );

scripts.js

// alerts 'Some string to translate'
alert( object_name.some_string);