Helping users to better decide wheter or not they should invest time reading the page the snippets on its search results belongs to, now since DMOZ is close, Google is advsing webmasters to provide more descriptive meta descriptions, "when adding more content to a page isn't an option." So, it can trigger better search result snippets that are more relevant to a search query, for people to click through and be satisfied with the page they land on.
Historically, snippets on search results pages came from 3 places: page content, meta description, and DMOZ listings. For over 10 years, Google relied on DMOZ for snippets, because of much higher quality than those in meta description, or were more descriptive than the pages.
Google explains how these snippets are generated until today, "The content of the page is an obvious a relevant choice for search queries result snippets. However, at times, content itself isn't the best source, and the logical fallback in those cases is the meta description — short blurbs precisely in a few words describing the content. But, there may be a time when a page doesn't have much textual content, or meta description is missing, or unrelated to page, or low in quality, in that case, the final fallback is DMOZ (Open Directory Project)."
Now, with DMOZ (ODP) is closed, thus the NOODP robots directive is already no-op, says Google, while it anser some general questions about meta descriptions on which it fallsback for generating snippets, explains, "A good meta descriptions are short blurbs that describe accurately the content of the page." Also, remember to include both a title and a meta description on both desktop and mobile pages.
What are the most common problems with meta descriptions?
Because meta descriptions are usually visible only to search engines and other software, webmasters sometimes forget about them, leaving them completely empty. It's also common, for the same reason, that the same meta description is used across multiple (and sometimes many) pages. On the flip side, it's also relatively common that the description is completely off-topic, low quality, or outright spammy. These issues tarnish our users' search experience, so we prefer to ignore such meta descriptions.
Is there a character limit for meta descriptions?
There's no limit on how long a meta description can be, but the search result snippets are truncated as needed, typically to fit the device width.
Can I prevent Google from using the page contents as snippet?
You can prevent Google from generating snippets altogether by specifying the "nosnippet" robots directive. There's no way to prevent using page contents as snippet while allowing other sources.