The Apple Watch will display notifications for any app that can push notifications to the watch. You can configure which can, by opening the Apple Watch app on your phone and tapping on the Notifications section. From here, you’ll be able to disable Notifications on the watch and control each app.
The app with the most options is Activity. This is where you can disable stand reminders, progress updates, indications that the goals are complete, summaries and of course achievement notifications.
Most of the other built-in apps can mirror an iPhone or be disabled. You can also manually control the built-in apps, which by default mirror alerts from your iPhone.
The Notifications Indicator option in that Apple Watch app will disable the whole red dot at the top of the screen thingy. I don’t recommend that. If you get a lot of notifications, the better thing to know is how to disable all the notifications at once. To do so, Force Touch to invoke the Clear All screen. Tap it and all those notifications get cleared at once. Now, hopefully you can find all the things you didn’t see.