Glances appear when you swipe up from the bottom of the Apple Watch. This shows relevant notifications, from both Apple and third-party apps. The watch vibrates when a new notification comes in. Different notifications have different UI’s as well as different buttons. Calendar notifications are presented with a red and white interface, for instance.
Apple is using a haptic feedback engine to power subtle buzzes to the phone, to alert you (but not people around you) to new notifications.
To dismiss the Glances view, just swipe down. It’s like a specialized version of iOS’s Notification Center, optimized for the smaller display of the watch.