the_content_limit() error

I’m getting this error :

PHP Fatal error: Call to undefined function the_content_limit() in /home/warpspee/public_html/fat-loss-resources/wp-content/themes/simplr/index.php on line 17

This is the code it is referring to

<center><h1>Fat Loss Resources</h1></center>

		<?php foreach( $posts as $post ) : setup_postdata( $post ); ?>

			<h2><a href="<?php the_permalink() ?>"><?php the_title(); ?></a></h2>
 <?php the_content_rss('', TRUE, '', 50); ?>

		<?php endforeach; ?>

I basically just want it to show an excerpt of each post

I’m a php dunce so this is probably an obvious fix