The meta description tag tells users what a given web page is about, right there in the search engine results, which helps them decide whether that page is worth visiting. To do that well, the tag needs to stay relevant to the page's actual content while also being intriguing, summative, and to the point, especially for users who haven't yet decided which result best matches what they need. It should also display fully and correctly on the results page, which generally means keeping its length between 55 and 165 characters.
Meta descriptions don't directly affect rankings; Google has said as much for years. Their real job is influencing click-through rate once a page is already ranking, since a compelling, specific description competing against generic ones nearby can be the deciding factor in which result actually gets clicked. It's also worth knowing that Google frequently rewrites the meta description shown in results, sometimes pulling a more relevant snippet from the page body instead, particularly when the written description doesn't closely match what a specific searcher typed. Writing a genuinely good one is still worth the effort even so, since it's the version Google defaults to whenever nothing more relevant is found on the page.
