The Apple Watch has a lot of health features built into it and also features designed to help encourage people to be more active. This includes things like reminding people to stand at least once every hour, reminding them to complete their activity circles, and so on. It can also alert people to things like an unnaturally high heart rate or irregular heart rate.
Now it seems in the latest version of iOS and watchOS, Apple has introduced a new health reminder tool in the form of warning users when their cardio levels are too low. This will be accomplished by using various sensors built into the watch, such as the heart rate sensor, GPS, and accelerometer to help determine how active the user is.
Typically, the Apple Watch would use the built-in VO2 max sensor (on newer models) to measure cardio levels, but this new feature means that in theory, it should be able to measure the cardio levels on users with older Apple Watches and also for times when users are unable to complete their high-intensity workouts.
Apple claims that based on studies, there is a link between low cardio fitness and a higher risk of health issues like heart diseases and high blood pressure, so these warnings should help encourage and prompt users to start getting in more exercise.