Generate

You can use wp_head action to add something to head section. You can change the output according your needs.

<?php
add_action('wp_head','add_meta_des');

function add_meta_des() {

    if(is_single())
    {       
        global $wp_query;
        $post = $wp_query->post;
        $page_id = $post->ID;
        $page_object = get_page( $page_id );
        $content = wp_trim_words($page_object->post_content,30);        
        $output="<meta name="description" content="".get_the_title()."--".$content.""/>";
        echo $output;
    }
}
?>