Step 1:

Open header.php form your current used theme folder.

Step 2:

Add below code in your file

<meta name=”description” content=”
<?php if (have_posts()&& is_single()):
while(have_posts()):the_post();
the_excerpt_rss(20,2);
endwhile;
else: ?>This is dynamic code .
<?php endif; ?> ”  />