Callback Example?

Can someone please share how the “callback” parameter works? Is it a PHP function callback, Javascript callback? I can’t seem to find a working example, and every test I’ve attempted doesn’t seem to fire. Here’s what my code looks like:

PHP including PHP callback

add_filter( 'infinite_scroll_options', 'mytheme_infinite_scroll_options' );

function mytheme_infinite_scroll_options( $options ){
	$options['callback'] = 'mytheme_infinite_scroll_callback';
	return $options;
}

function mytheme_infinite_scroll_callback( $vars = array() ){
	echo '<script type="text/javascript">alert(\'php callback\');</script>';
	return $vars;
}

JS Version

function mytheme_infinite_scroll_callback(){
	alert('js callback');
}

I don’t get either alert.

https://www.ads-software.com/plugins/infinite-scroll/