I write function in functions.php such as this

function out() {
  $s = 'End of the post, thanks for reading';
  return $s;
}

add_filter('the_content','out');

and that i expect this to become fetched in the finish from the publish, after entry content. But all it will is the fact that publish entry ( exactly what the_content results) isn't proven, and that i only get 'End from the publish, thank you for reading'.

What shall we be held doing wrong?

Do this:

 function out($content) {
     return $content." End of the post, thanks for reading";
}

add_filter( 'the_content', 'out' );

Are you able to do this

function out($content) {
  $content .= '<br>End of the post, thanks for reading';
  return $content;
}

add_filter('the_content','out');