function sample_function ( $content ) {

  $content .= 'test';

  return $content;

  }


add_filter(
'the_content', /*the function tag/id for the WordPress Plugin API function*/

'sample_function', /*callback function that you are going to run the filter through*/
);

/* SUMMARY = this takes the post content and appends test to it. i.e filters the content changin it to a new form. */