The meta description is an HTML tag holding a one- or two-sentence summary of the page. Search engines often use it as the snippet under the result, but they may pick other text from the page instead if it fits the query better.
It is not a direct ranking factor, but a clear description can improve clicks. Write it for people: say what the page offers and why it is worth opening, use natural wording that reflects the search intent, and keep it unique for each page. Length is cut off by display width, so put the main point first.
Do not copy the same description across many pages, and do not expect it to appear exactly as written. It works alongside the title tag and the page's content.
Related: Search Engine Optimization, H1, Technical SEO.