A MySense event is a change in pattern or activity in your daily routine. If there are any disparities, a notification is sent to you and your Circle when MySense notices a change over time. This could be in your routine, your activity levels, or something outside of 'normal' for you based on your data over time. They may regard also the status of your sensors or home hub.
These events are viewable in the MySense app. You can get more information and guidance about the event within the app by tapping the notification on your device.