Note the function assigned to the show: option in the call to tabs. This adds text, which is meant to be visible only to screen readers, to the tabs list indicating which tab is currently active. We use CSS to move text outside the viewport, but leave it unhidden in the DOM. This visually hides it, but allows screen readers to access it as they would any other content.
Here is a technique, which I have not tried, for visually hiding content
while still making it available to assistive technologies such as screen readers.
It uses something called CSS Clip.