What does 925 actually mean?

Sterling silver is 925 parts per thousand silver — the remaining 75 parts are copper, added because pure silver is too soft to hold a clasp. Anything below 925 isn't legally allowed to be called sterling in the EU. Anything above (e.g. Britannia 958) is rare and expensive. Our hallmark on every piece reads 925 because that's the workhorse alloy that's been used for the past 700 years for one reason: it works.