Your watch can measure your heart rate throughout the day to give you an overview of your activity and recovery level.
The Heart rate Tile provides a quick way to check your current heart rate and the 24 hour graph visualizes how your heart rate changes throughout the day — when you rest and when you are active. While the graph updates every 10 minutes, your current heart rate is measured every second to give you up to date feedback.
You can also go to your Diary > Daily activity to see your daily heart rate information.