[ Thanks to IdaAshley for this link.
]
The initial RAM disk (initrd) is an initial root file system
that is mounted prior to when the real root file system is
available. The initrd is bound to the kernel and loaded as part of
the kernel boot procedure. The kernel then mounts this initrd as
part of the two-stage boot process to load the modules to make the
real file systems available and get at the real root file
system.“The initrd contains a minimal set of directories and
executables to achieve this, such as the insmod tool to install
kernel modules into the kernel…”