Meta Description is a feature inside the hidden programming of every web page. It allows 150 characters to describe your web page.
This description is read into the search engines and displayed on the Search Engine Results Page as the 2 lines of text below the website page title. Since you're probably not looking at a SERP right now, let's describe this.
The SERP shows the title of the web page as blue underlined text. Right below that, there are usually 2, sometimes 3, lines of black text, and right below the black text is a green website address. That black text is the Meta Description, and you (mostly) have control of what shows up there.
The functionality of the meta description has changed over the years, and a lot of out-dated information is still available online. In fact, when it comes to the meta description and the meta keywords there is more out-dated information online than updated and factual information.
Many new website owners will believe the majority of the information they find. One person said it, two people said it, 25 different websites said it so it must be true that meta descriptions are used for search engine ranking and optimization.
Actually, that's totally wrong and Google has published many videos to debunk the old ways where web programmers would repeat key phrases over and over again the in the descriptions.
Several years ago, it was common to see SERPs riddled with "engagement ring store, bridal ring store, engagement rings store, wedding rings, engagement rings, gold engagement rings" as web programmers tried to stuff variations of keyword phrases into the description hoping to better their ranking.
Well, Google and Bing are smarter now. That technique doesn't work. In fact, if you even try that technique, Google will notice it and instead they well take a snippet from the body of your page and use it in place of your stuffed keywords.
So what's the lesson for today?
Simply write real descriptions of what you will find on every page of your site and put those into your Meta Descriptions. Think in terms of short sales messages and use that to tell prospective customers exactly what they will find when they click.