Zeta is the name of a virtual platform, or architecture.
There are specifications (see the handbook), and a basic, not-yet-optimised emulator (cpu, ram, clock, mainboard). The architecture is simple, barely having the minimum needed to run linux with MMU.
The purpose is to play with the toolchain and the linux kernel, to learn more about those core technologies. Binutils and gcc are already ported at 80%. (No static lib nor PIC code yet, userland compilation untested.) Using those tools (as "cross-compilers"), the real work can now begin : porting linux to Zeta. This is still work in progress. This kernel is called ZetaLinux.