The Raspberry Pi is really a VideoCore IV processor with an ARM bolted on the side. At power on, a boot ROM loads an embedded OS image into the VC4 processor. That then sets up the hardware, powers up the ARM, and loads the OS image into that. The VC4 then continues running in the background servicing RPCs from the ARM.
What this is is a replacement VC4 operating system image which just fires up the ARM with an embedded image, sets it running, and then halts (assuming I'm understanding the docs correctly); so it can't (yet) actually load an OS into the ARM from disk or respond to RPCs etc.
In practical terms, this is the biggest, most difficult step forward towards running an almost completely open system on the RPi. (It could also be used as the core of a proper operating system running on the VC4 itself, which could be rather interesting.)
What this is is a replacement VC4 operating system image which just fires up the ARM with an embedded image, sets it running, and then halts (assuming I'm understanding the docs correctly); so it can't (yet) actually load an OS into the ARM from disk or respond to RPCs etc.
In practical terms, this is the biggest, most difficult step forward towards running an almost completely open system on the RPi. (It could also be used as the core of a proper operating system running on the VC4 itself, which could be rather interesting.)