— What is FOBOS?
FOBOS is the project of a portable and operating system independent filesystem interface.
You should ask, what is a filesystem interface.
From wikipedia: "A filesystem API is an application programming interface that allow developers to add support of a filesystem to an operating system without needing the second to know anything about what filesystem is it or how it works."

— How does it work?
At the moment it works in userspace, that means, it works as any other application providing you an user interface to access filesystems.
In a future can be made in kernel-space, inside a kernel.
It is divided in three principal sections, the shell, the helper and the modules.
The shell is the user interface, and can be a graphical program or a command line or, in kernel-space, a translator between kernel's native interface and FOBOS.
The helper is the code that abstracts the modules from the operating system, making it operating system independent.
The modules are the ones that really handles the filesystems and partition maps, as user requests using the shell, and calling the helper to access the hardware.

— How can I help?
You can help making a module for your favourite filesystem or partition map, or you can help contributing to FOBOS code directly.
Just ask to the chief developer for CVS access and a developer identifier to start working.