can’t understand _e function well

$domain is the unique identifier for retrieving translated strings.

It is set in functions.php

<?php load_theme_textdomain( $domain, $path ) ?>

ex:

load_theme_textdomain('my_theme', get_template_directory() . '/languages');

<?php _e( 'Translate this text', 'my_theme' ) ?>

If you don’t have a translation file but want’t to make it translatable just define the textdomain.

load_theme_textdomain('my_theme');