CMB2

First download the cmb2 files and paste the folder (e.g. cmb2) theme in the root folder of our theme. Create a functions.php file inside that folder. We will write all metabox codes in this file.

include the code in main functions.php

require_once('cmb/init.php');
require_once('cmb/functions.php');

write the following code inside cmb2/functions.php file

function amra_metabox_banabo(array $ourmetabox){
    $ourmetabox[] = array(
        'id' => 'first-section',
        'title' => 'what is this post is all about',
        'object_types' => array('page'),
        'fields' => array(
            array(
                'name' => 'type your opinion',
                'type' => 'text',
                'id' => 'our-opinion'
            )
        )
        
    );
    
    return $ourmetabox;
}

add_filter('cmb2_meta_boxes', 'my_meta');

and the write the following code inside while loop of the place where we want to show the value of meta box

<?php echo get_post_meta(get_the_ID(), 'our-opinion', true); ?>

CMB2 for specific pages:

function komola_cmb(array $ourmetabox){
    $ourmetabox[] = array(
        'id' => 'first-section',
        'title' => 'My cmb2 meta box info',
        'object_types' => array('page'),
		
		'show_on'   => array(
				'key'       => 'page-template',
				'value'     => 'service.php'
		),
		
        'fields' => array(
            array(
                'name' => 'type your opinion',
                'type' => 'text',
                'id' => 'our-opinion'
            )
        )
    );
    
    return $ourmetabox;
}
 
add_filter('cmb2_meta_boxes', 'komola_cmb');

show_on details.