What does this mean?
The meta description is considered important to help users quickly understand what content they can expect to find on the page, when clicking through from the search engine results page. Well written meta descriptions typically achieve a better click-through-rate.
If the meta description is empty, this represents a missed optimization opportunity. Furthermore, this means that search engines will write the meta description for you, sometimes with pretty disastrous results.
What to do?
Create well-written meta descriptions that match the content on your page.
Learn more: https://moz.com/learn/seo/meta-description