This feature transmits your flight data information on every control loop iteration over a serial port to an external logging device to be recorded.
firmware main info here : https://github.com/cleanflight/blackbox-firmware
After your flight, you can process the resulting logs on your computer to either turn them into CSV (comma-separated values) or render your flight log as a video using the tools
blackbox_render. Those tools can be found in this repository:
You can also view your flight logs using your web browser with the interactive log viewer: