The watch world is full of jargon that's tough to understand. Chronometer might sound like a mouthful, but it's just a fancy word for "really, really precise watch."
The word chronometer goes back to the early eighteenth century, when an English clockmaker called Jeremy Thacker invented a vacuum-sealed clock. Without air resistance, it was extremely accurate; Thacker dubbed his creation a chronometer. The name was later used a few decades later to describe marine chronometers—clocks suspended in gyroscopic boxes that helped ships determine longitude and traverse the world's oceans. Today you can wear their precision-crazed successors on your wrist.