The description is a summary of what the page is about.
A page's description might be a sentence or two or even a short paragraph (whereas a page's title may be a few words or a phrase).
It's a page metadata that:
You can modify the description via the metadata manager.
This component will also create the following description HTML meta header for the below external services.
<meta name="description" content="The best page on the universe">
<meta property="og:description" content="The best page on the universe">
<meta name="twitter:description" content="The best page on the universe">
The description is not set specifically for the Article Google Rich Result Card, they used the meta description tag.