it’s only a function to get the caption of a featured image, but this means not that it’s always in use for every featured image. We made the demo websites to show how the theme work, and if there is no caption on / below the image, then we don’t used it in the theme.
But you can use this function to return the caption below the image, i.e.
< ?php echo get_post_thumbnail_caption(); ?>
For future posts: don’t show full functions or other php code in the forum. Thank you.