the second indicator

Sorry, I don’t know the jargon or I would probably either be able to find an answer with Google or just know the answer.
I have set up a child theme for TwentyEleven.
I intend to change all the templates.
Something I have noticed – not just in TwentyEleven but in other themes as well – is a self-referential note in the code. I can’t explain, here’s an example (from 404.php):

<h1 class=”entry-title”><?php _e( ‘This is somewhat embarrassing, isn’t it?’, ‘twentyeleven’ ); ?></h1>

Note how it says ‘twentyeleven’ there at the end? This seems to be in every line of code.
In the past, when modifying these lines, I have sometimes omitted the name of the theme, as well as the quotes and comma, with no effect at all. (This was also before I learned about child themes.)

What I would like to know is – what is that? What does it do? Is it necessary? If I am modifying templates in my child theme, should I change it to be twentyeleven-child? Or leave it? Or delete it?

Thanks!