The firmware is set up to use the Segger RTT logging and debugging. Debug output is written using NRF_LOG et al and the makefile is set up to use the RTT logger.
Download the JLink tools at Segger
Debugging in macOS
Launch the JLinkExe in one terminal:
$ JLinkExe -if swd -device nrf52 -speed 4000
connect to connect to the device.
JLinkRTTClient command in a separate terminal to view the RTT log. When you redeploy you have to relaunch the
Download an improved JLinkRTTClient command from https://github.com/stalehd/jlinklogviewer. It will reconnect to the JLinkExe server whenever it is restarted.