Display the featured image of a post in a tag.
Additionally if you want to grab a specific size for the featured image you can fill out the second argument with an image size. Also can add class onto image.
echo get_the_post_thumbnail( get_the_ID(), 'medium', array('class' => 'img-fluid') );
Return the URL of the featured post image.
echo get_the_post_thumbnail_url( get_the_ID(), 'medium' );