???? if you want to add meta tag description you should go to editor > header.php then add your meta tag between <title></title> or you can use all in one seo pack for easy ways.
To add meta tags to your site, simply add them to the header.php template file in your WordPress Theme, specifically in the head section near the link for the style sheet. At the top you will see the DOCTYPE tag and below that you will see a couple more tags and then the <title> tag, looking something like this: <title><?php bloginfo('name'); ?><?php wp_title(); ?></title> Below this line you can add your meta tags. You can add meta tag information such as the content, language, author, contact, and copyright, since these are basically the same on every page of your site.
Plugins are best options to add meta tag below are list of some good SEO plugins All in One SEO Pack Automatic SEO Links Platinum SEO Plugin
Just install All In One SEO plugin, Once installed this, you can add seperate Title, Keywords, Description for each page or posts.