The Meta Description tag allows you to defined an hidden description in a page. This description is not directly displayed on the page, this is what we call a Meta data.
The description tag is used by Google in the SERP:
The Meta Description is defined in the Head of your HTML code:
<meta name="description" content="This is an example of a meta description. This will often show up in search results.">
You can, thanks to the Meta Description, influence the click-through rate on your link in the SERP. You need to carefully choose your description and not to write for keywords but for users. Opt for an attractive and dynamic description to attract users.
Unfortunately, Google doesn't take into account your Meta Description in all cases. If your description is too long, Google will reduce it.
The perfect length for the description is between 50 and 60 characters.
Of course, ensure your Meta Description matches with your page.
You will find all your Meta Descriptions to optimized in the tab Pages > Content