This creates a link to the next image attached to the current post. Whenever a series of images are linked to the attachment page, it will put a 'next image link' with the images when viewed in the attachment page.
Typically uses in attachment.php. In the WordPress default theme Twenty Eleven and Twenty Twelve, it is used in image.php.
<?php next_image_link( $size, $text ); ?>
<?php next_image_link(); ?>
<div class="alignright"><?php next_image_link(); ?></div>
See also previous_image_link()