The meta description is a snippet of up to about 155 characters which summarizes a page’s content. Search engines show it in search results mostly when the searched-for phrase is within the description. So optimizing it is crucial for on-page SEO.
If you do not write a description, Google will create one for you through words you already have on the page. (which is not always an accurate description for your page)
Unfortunately, we can’t fully control what Google displays in the search results. Sometimes it decides to show the meta description, and sometimes it grabs some sentences of your copy. Either way, your best bet is to keep it short. That way, if Google does decide to show the meta description you’ve written, it will be shown fully.
Check out some additional materials on Meta Descriptions.