Meta descriptions have no fixed character limit. What they have is a display width, and Google truncates whatever exceeds it — usually somewhere around 155 to 160 characters on desktop, and less on mobile.
What the checker reports
It measures a batch of descriptions against typical display limits and shows which ones fall outside. Both directions matter: too long gets cut mid-sentence, and too short wastes space that could have earned the click.
Why descriptions still deserve the effort
They are not a ranking factor. They are a click-through factor, and click-through is what turns a ranking into traffic. A page sitting at position four with a compelling description can outperform position two with a truncated one.
The rewrite that follows
Put the reason to click in the first hundred characters. Anything after that is a bonus that may or may not survive truncation.
Try it: Meta Description Length Checker on SeoWolf's Notepad.