This package contains nbd-server. It is the server backend for the nbd
network block device driver that's in the Linux kernel.
nbd can be used to have a filesystem stored on another machine. It does
provide a block device, not a file system; so unless you put a
clustering filesystem on top of it, you can't access it simultaneously
from more than one client. Use NFS or a real cluster FS (such as
ocfs2) if you want to do this. nbd-server can export a file (which may
contain a filesystem image) or a partition. Swapping over nbd is
possible as well, though it's said not to be safe against OOM and
should not be used for that case. nbd-server also has a copy-on-write
mode where changes are saved to a separate file and thrown away when
the connection closes.
The package also contains the nbd-client tools, which you need to
configure the nbd devices on the client side.