Definitions for "Initrd"
a small filesystem image and/or bootsplash that your bootloader can load and your kernel can boot to before any other filesystems are mounted
The initial ramdisk, or initrd is a temporary file system used by the Linux kernel during boot. The initrd is typically used for making preparations before the real root file system can be mounted. When specified, the boot loader loads the ramdisk into memory and passes it on to the kernel, which temporarily mounts it as root and executes the /linuxrc executable.