If it’s the same as mine, it measures your heart rate overnight after you settle in. Then, when you wake up, it tells you your “resting heart rate” number.

It can be an indicator of short-term stress if it is higher than normal, for example. But the interesting part is the trend over time. As you get more fit, the average drops

Unless you are very fit 40bpm is very low (bradycardia) and could be a medical issue.