The following article applies to iOS only.

You can connect a Bluetooth heart rate sensor to Seconds directly within Settings > Heart Rate Settings. First tap Discover Devices, then select your sensor when it appears. Now when you start a timer Seconds will attempt to discover this sensor.

Though it is possible to connect a Bluetooth heart rate sensor directly to Seconds in the Settings if you are using an iPhone we recommend you connect the sensor in the device settings and let it write directly to Apple Health. Seconds will then pick up the data from there.

Apple Watch is not a Bluetooth heart rate sensor and as such will not be picked up in the Heart Rate Settings options. It is still possible to use Apple Watch to detect your heart rate by running a workout in the Workouts app while using Seconds on the iPhone. When we update Seconds to work on both devices simultaneously this will not longer be required.

Once you have connected your heart rate sensor you will also need to reconfigure you timer display to show this information. This can be done in Settings > Timer Settings > Configure Timer Display where you can choose a range of values including all the different heart rate options.
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The heart rate zones are based upon your max heart rate which will require you date of birth to be entered within Settings > Personal Details. The zones are then defined as:

Your maximum heart rate is a crude calculation based upon the calculation: 220 - Age. You can define you own observed maximum heart rate if this value is not accurate enough for you.