Remind You To Stand
Part of the Apple Watch activity monitor feature is a reminder to stand up and move. Sitting comes with health risks and the Apple Watch can send you an alert when you sit for too long.
You'll get this notification if you haven't stood for about an hour and your goal is to fill the Stand ring by standing for at least one minute in 12 hours each day.
This is on by default, so you don't need to set it up or turn on the Stand reminders.