And I have already understood these.

<div class="skip-link"><a class="assistive-text" href="#content"><?php _e( 'Skip to primary content', 'twentyeleven' ); ?></a></div>
<div class="skip-link"><a class="assistive-text" href="#secondary"><?php _e( 'Skip to secondary content', 'twentyeleven' ); ?></a></div>

It shows up as a link when I click TAB.But I still don’t understand this one
<h3 class="assistive-text"><?php _e( 'Main menu', 'twentyeleven' ); ?></h3>

This code:

<h3 class="assistive-text"><?php _e( 'Main menu', 'twentyeleven' ); ?></h3>

Is a heading used to help people identify the main navigation element. This theme is quite old and there are better ways to do this nowadays, such as using the ‘aria-label’ attribute on the navigation element

It should not however show on tab, because it’s not a tabbable element. Tabbable elements are interactive elements and this is just a heading.

Either there’s a bug on your site showing this on tab, or you’re looking at the wrong thing.

This code is the one that will show up when I click “TAB”.It’s from theme twentyeleven.

<div class="skip-link"><a class="assistive-text" href="#content"><?php _e( 'Skip to primary content', 'twentyeleven' ); ?></a></div>
<div class="skip-link"><a class="assistive-text" href="#secondary"><?php _e( 'Skip to secondary content', 'twentyeleven' ); ?></a></div>

But thanks for your help I have understood now.