Saying Y here will allow you to use a portion of your RAM memory as
a block device, so that you can make file systems on it, read and
write to it and do all the other things that you can do with normal
block devices (such as hard drives). It is usually used to load and
store a copy of a minimal root file system off of a floppy into RAM
during the initial install of Linux.

Note that the kernel command line option "ramdisk=XX" is now obsolete.
For details, read Documentation/blockdev/ramdisk.txt.

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

Most normal users won't need the RAM disk functionality, and can
thus say N here.

source code:

drivers/block//brd.c