Click-through copy for the search snippet, with a live counter against Google's truncation point.
A meta description is the snippet under your link in the search result. It is not a ranking factor, and Google rewrites it most of the time. It is still worth writing.
Because when Google doesn't rewrite it, the alternative is worse: with no description at all, the snippet becomes whatever fragment of your page happened to match the query. That is your pitch, chosen at random. Writing one is how you get a say in the common case.
Google cuts the snippet at roughly 155-160 characters on desktop and less on mobile. Nothing bad happens if you go over — the tail just doesn't get read. The counter here marks that point so the part that matters lands before it.
Meta description length, and what a missing one costs — the longer version, with sources.