In more detail
The tag sits in a page's <head> as <meta name="description" content="…">. Google's documentation on controlling snippets explains that it creates snippets automatically, mainly from page content, and may use the meta description "if it gives users a more accurate description of the page than would be possible purely from the on-page content".
Google's best practices include:
- Write a unique description for each page, rather than reusing one across the site
- Summarise what the page offers, in a way that's informative and relevant
- Include useful specifics, such as price, author or opening hours, where relevant
- Don't stuff it with keywords
Google doesn't set a character limit. Snippets are truncated to fit the device, so it's sensible to put the most important information first. On sites with thousands of pages, descriptions can be generated from page data, such as a product's name, price and key features.
Site owners can limit snippet length or stop snippets entirely with the max-snippet and nosnippet controls described in the same documentation.
Why it matters when hiring an agency
Meta descriptions are cheap to improve and can help click-through rates, especially on pages with many impressions but few clicks. They won't move rankings on their own.
Be cautious of audits that flag "missing meta descriptions" across every page and propose to rewrite them all by hand. Prioritise pages that matter commercially and receive impressions. Ask the agency how it will check results in Search Console after changes, bearing in mind that Google may still choose its own snippet.