A variation on a classic tool. There are two little prongs on the backside of the tool which you settle betwixt any two given links, with the dial set to 0, wherever the tool naturally rests. Then turn the little silver dial clockwise until you feel a light pressure. Take a reading off of the little pointed nub, the readings run from .25 (25% wear) to 1.0 (100% wear). If the dial just spins infinitely you're past 100% wear. Chain manufacturers recommend replacing your chain at .6 or 60% wear. For 1x drivetrains, that's probably a good idea. You can possibly run your chainring and cassette for a bit longer that way. If you just ride the chain past full wear, odds are you've really worn through the chainring and cassette as well, and putting a fresh chain on worn down cogs doesn't end well. The teeth of the cogs wear unevenly into little shark tooth shaped fins, and the new chain has trouble releasing off of that fin. For 2x, 3x systems, you can typically get 3 or more chains worth out of the rest of the drivetrain, since the wear is spread out across more cogs.