Plausible's first-party cookie is used to differentiate between a new visitor and a returning visitor. This cookie contains a random identifier that is sent along with each pageview request. This allows Plausible to aggregate and show unique visitors and total pageviews separately.
Plausible is an open source alternative to Google Analytics that respects user's privacy. You can read about Plausible's Data Policy here].
Plausible is totally transparent about how it process the data it collects and, as such, is open source. github.com/plausible-insights/plausible
You can identity the Plausible cookie in use at Penmorvah.co.uk by the following name: plausible_user.