The meta description is a sentence or two describing the page. It does not appear on the page itself; it lives in the code, and Google often uses it as the grey text under your title in search results.
It does not directly affect rankings, but it does affect whether people click. Google shows roughly the first 150 to 160 characters on most screens and frequently replaces the description with text from the page if it thinks that answers the search better.
A good one reads like a quick pitch: what you do, where, and one reason to pick you, such as fast response times or being fully insured.
Ranking fourth with a clear, specific description can win more calls than ranking second with a vague one. It is a few minutes of writing per page, and it is one of the easiest places to make your listing look more trustworthy than the competition.