Apple takes your heart rate reading in the background every few minutes when wearing your Apple Watch, however some people would like to have it recording continually.
There are some downsides to running the heart rate constantly which mainly is the affect on the battery throughout the day.
How to Track
Cardiogram is a free app that offers premium in app purchases, however, their free tier is more than sufficient for most people.
Once installed and you have granted it permission to your health data you open the app on your watch.
The watch will now continually record your heart rate until you stop it.