subscribe by mail plugin: ajax-loader.gif

on my homepage: I inserted the subscription widget (at the bottom of the sidebar).
The ajax loader gif is displayed but I don't know why. Is there an ajax loading process ongoing or does my theme not support ajax calls? what is the following line of code for in the widget.php template?
<img src="<?php echo INCSUB_SBE_ASSETS_URL . 'images/ajax-loader.gif'; ?>" class="subscribe-by-email-loader"/>
thanks for your help.