ZFSBootMenu is a bootloader for Linux kernels residing in a ZFS pool. It basically searches your ZFS pools for datasets that look like a Linux root filesystem and it presents them to you as options to boot from (source). Once an OS has been selected to boot, it lets go of the pool and boots the OS, at which point it's the OS's responsibility to mount the ZFS pool(s) (source).
All this tool does it provide a convenient means to load up your kernel from a zpool and boot it, that's it (and some nice pre-boot tools).