WatchOS is the operating system (or software) that makes your Apple Watch work! It's like the brain of your watch that controls everything it can do.
Just like your mom or dad sometimes tell you what you can and can't do, WatchOS tells your watch what it can and can't do. It makes sure all the buttons and features work properly, and helps the watch connect to other devices like your iPhone or iPad.
WatchOS includes a lot of cool things you can do on your watch, like checking the weather, answering phone calls, sending text messages, tracking your workouts, and even paying for things with Apple Pay! It also has lots of fun watch faces you can choose from to make your watch look exactly how you want it.
Without WatchOS, your Apple Watch wouldn't be able to do anything – it would just be a fancy accessory on your wrist. But with WatchOS, you can use your watch to do all sorts of cool stuff, and even make your life a little bit easier!