What is __(arguments) in my functions.php

Hi guys.

I have been handed the work of previous developers and so I am analysing the functions.php in my Circles child theme.

Basically there is this code, which I know what the function does except for the __() part:

register_post_type( 'services',
		array(
			'labels' =>
                array(
                    'name' => __( 'Services' , 'circles'),
                    'singular_name' => __( 'Service' , 'circles')
                ),
            'public' => true,
            'has_archive' => false,
            'rewrite' => array( 'slug' => 'services' ),
            'supports' => array('title',
                'editor',
                //'author',
                'thumbnail',
                //'excerpt',
                //'comments'
                'page-attributes'
            )
		)
	);

I know that in PHP __construct() refers to a constructor of class but am unsure of the above syntax in WordPress.

Thank you.
Regards