Root is your the second partition of you SD card (first is the boot usually).
It contains the operating system and the programs (settings, logs, etc.)
The latest official software has some bugs, this is the reason why you can only see root %.
I made some bugfixes and tweaks in the code.
Read a little bit over here: [Fixes and New Features] Fixed disk usage display, disk temperatures, disk and network activity, fan speed percentage display on OLED display, improved uptime format