Saying Y here will allow your computer to be a client for network
block devices, i.e. it will be able to use block devices exported by
servers (mount file systems on them etc.). Communication between
client and server works over TCP/IP networking, but to the client
program this is hidden: it looks like a regular local file access to
a block device special file such as /dev/nd0.

Network block devices also allows you to run a block-device in
userland (making server and client physically the same computer,
communicating using the loopback network device).

Read <file:Documentation/admin-guide/blockdev/nbd.rst> for more information,
especially about where to find the server code, which runs in user
space and does not need special kernel support.

Note that this has nothing to do with the network file systems NFS
or Coda; you can say N here even if you intend to use NFS or Coda.

To compile this driver as a module, choose M here: the
module will be called nbd.

If unsure, say N.

source code:

drivers/block//nbd.c