The short summary shown beneath your title in search results. It is not a ranking factor, but it heavily influences whether anyone clicks.
Google rewrites meta descriptions for a large share of results, pulling a more relevant sentence from the page instead. That is not a reason to skip it — a well-written one is kept more often than a poor one, and for brand searches it is almost always kept.
Treat it as ad copy. You are competing with nine other results, and the one that most obviously answers the question wins the click regardless of position.
There is a free calculator on this site for exactly this.
A free consult is twenty minutes and there is no pitch deck at the end of it.