There are two ways to contextualize your heart rate in terms of intensity: % of Max Heart Rate and the Karvonen formula. The app supports both of these values, you can toggle between the two under MORE>HEART RATE INFO in the app.
% of MaxHR gives you an intensity value as a percentage between 0 and your custom Maximum Heart Rate. If your MaxHR is 200, for example, and your heart rate is 100 you are at 50% intensity.
The Karvonen formula, on the other hand, gives you an intensity value as a percentage between your resting heart rate and your custom Max HR. This means that if your resting heart rate is 60 BPM 60 will correspond to 0%.