Need to show post summary in sidebar

Question

Easy way is use – excerpt() else you can also use wp_trim_words( get_the_content(), 40, '...' );

In a excerpt(), You have to add content on excerpt in backend. While wp_trim_words will trim words from content.

In sidebar case you have to add shortcode into functions.php file which uses in sidebar widget(if you are using widgets).

function test_short(){
    ob_start();
    global $post;
    echo wp_trim_words( get_the_content($post->ID), 40, '...' );
    return ob_get_clean();
}

add_shortcode('test_short', 'test_short');