Meta Tag “description”

Do not add meta tags in post editor. Best way to do is use SEO plugins.

Best plugin for SEO is Yoast Seo

Or add this code in your themes header.php just after title tag.

<meta name="description" content="<?php if ( is_single() ) {
        single_post_title('', true); 
    } else {
        bloginfo('name'); echo " - "; bloginfo('description');
    }
    ?>" />