Adding code using functions.php in child theme

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.