DragonFlyBSD has a concept of "vkernel" for allowing kernels to be loaded from user-space for testing purposes. DragonFlyBSD developers can load a vkernel from user-space for debugging or testing the kernel. With it being done from user-space it's quick and easy for testing and avoids the needs for reboots, etc. For those unfamiliar with the DragonFlyBSD vkernel concept, see this handbook page or the vkernel man page.
What the new DIRFS file-system is for comes down to being a psuedo file-system that can mount the host's directories into the virtual kernel. DIRFS runs from the virtual kernel's VFS layer but uses syscalls to communicate with the host kernel about data found on the host file-system.
DIRFS is currently considered an experimental feature that's been committed to the DragonFlyBSD kernel. For more information see the Git commit message introducing the new file-system.