You get the $content
as a parameter for your function, and you should not echo anything in that function. Return a string instead.
Example:
add_filter( 'the_content', 'app_bits');
function app_bits( $content )
{
if ( ! is_singular() )
{
return $content;
}
$extra="<div style="text-align: center;">custom code goes here</div>";
return $content . $extra;
}
See How to return loop contents for an example showing how to return a rather complex string with markup.