The summary text Google shows below your page title in search results.
The meta description is a short HTML tag (<meta name="description">) that tells Google what the page is about. Google may or may not show it verbatim, but it often does. A good meta description is 150-160 characters, describes the page value, and includes the target keyword. It doesn't directly affect ranking, but it heavily affects click-through rate.
You spent months ranking on page 1. The meta description is whether anyone actually clicks. Treat it as ad copy for organic. Rewrite weak ones; test variants; improve CTR as a direct lift to traffic.