Theme name passed into some of the functions?

I use a theme called vigilance

I am starting to build my own themes and I notice in this them a lot of functions will have the theme name vigilance passed into the functions like this one below…

<?php _e( 'This post is password protected. Enter the password to view comments.', 'vigilance' ); ?>

Notice the theme name is passed as the second parameter, I am trying to figure out why? I have not found anything supporting this in the docs yet?