This HTML file is located on the MAINTENANCE drive (the mode entered when you hold down the reset/power button on your micro:bit as you plug it in). It is locked and so cannot be edited. The file contains information about the micro:bit, including the interface/bootloader version (described in DETAILS.TXT)
For example, the file may contain a link such as
https://microbit.org/device/?id=0000&v=0255&bl=1
This link currently directs you to the user guide on the microbit.org website, but could be used in future to provide specific guidance on the board or the firmware that is being used.