Comment on WordPress SEO Meta Tags 2016 by SEO Dave.
By default the description meta tag in Stallion uses an excerpt for a Post. If when you write a Post (or edit it later) you give it a custom excerpt (you can write an excerpt for any post you like) that excerpt will be used. If not an automated excerpt will be generated using the first X characters of the post (X characters is determined on the Stallion SEO Advanced options page). At the end will also be the blog description if you have a blog description.
The meta description has no ranking value in Google, so this is about click through rates from Google searches. If the meta description tags reads well (like a well written ad) it might increase the click through rate (again will make no difference to the actual ranking) compared to a randomly generated meta description.
There’s a very strong argument if you aren’t writing custom excerpts for most of your posts you might as well turn of the automated meta description tag and let Google determine a description based on what the user was searching for. I’m tending to do this on most of my sites now (most of my sites lack a meta description tag and meta keywords tag) as have better things to do than write ad like excerpts for thousands of posts (I’m not that good at writing ads to be honest :-)), however if I owned just one site I’d write custom ad like excerpts.