why does the add_action(‘the_content’) overwrite my web page

I’m new to wordpress and I’m attempting to show some textual content on particular pages on my web site. My drawback is that my plugin replaces my present content material on that web page with the one on my plugin. How can I make the plugin not substitute the content material on the web page.

add_action( 'the_content', "displayNewsSlider");
perform displayNewsSlider(){
  if (is_page('sample-page')){
  echo "plugin content material 1";
  echo "plugin content material 2";
  }
}