The meta description is the copy that Google uses to describe your page in its search results. I've added a screenshot below to demonstrate this.
It's essentially a description of the page that sits within the code; it's not displayed anywhere on your actual web page.
The meta description is important because it summarises your page's content for users looking to click on one of the websites Google has listed in front of them.
Therefore you want to make sure it is descriptive, accurate and enticing! Plus you want to add your main keyword (if it fits in naturally) as Google will highlight this in bold and it improves your click-through rate.
Also, try to add copy that will draw customers eyes and get them to click your website listing, as opposed to your competitors.
You can add USPs and call to actions, like 'Free delivery' or 'Visit our website today' to improve your click-through rate.
Important! The ideal length is between 140-155 characters long. Use a service like https://wordcounter.net/ to keep track of length.