I might be wrong somewhere, but think of Hurd server as an orchestrator, which runs atop the GNU/Mach microkernel, as opposed to monolithic kernels like BSD and Linux.
It’s an OS that’s still not in existence - I mean, well, technically, it is, the QEMU images, as well as Debian/Arch ISOs are available on the internet, but the Mach microkernel is slow, which is a blocker, so they’re looking into alternative ones.