<?php if (is_home()) { ?>
 <meta name="description" content="Your description for homepage..." />
<?php } else { ?>
 <meta name="description" content="Description for other pages..." />
<?php } ?>

<?php
the_post();
if (  is_home()  ) { ?>
<meta name="description" content="YOUR DESCRIPTION" />
<?php } elseif (is_single()) { ?>
<meta name="description" content="<?php the_excerpt(); ?>" />
<?php } ?>

<?php
function add_meta_home() {
    if (is_home() || is_front_page()) { ?>
        <meta name="description" content="Lorem ipsum dolor sit amet."/>
        <?php
        }
    }
add_action('wp_head', 'add_meta_home');
?>