You can use first few lines of post content and add these to meta description tags to website head section automatically.
Here is how you do that.
// add meta description tag
function wcs_add_meta_description_tag() {
global $post;
if ( is_single() ) {
$meta = strip_tags( $post->post_content );
$meta = strip_shortcodes( $post->post_content );
$meta = str_replace( array("\n", "\r", "\t"), ' ', $meta );
$meta = mb_substr( $meta, 0, 125, 'utf8' );
echo '<meta name="description" content="' . $meta . '" />' . "\n";
}
}
add_action( 'wp_head', 'wcs_add_meta_description_tag' , 2 );
Ofcourse You can change this code to use post excerpt for meta description tag.