Maybe you mean "initrd"?
initrd is the an initial ram disk archive. It is related to the linux kernel modules and the boot sequence.
Basically, linux uses driver modules to control diverent pieces of hardware in your computer. It loads the modules from the disk when they are needed. The problem to load the modules from the disk is that for that you need to access the disk with the disk and fs drivers, and those drivers are modules on the disk. The initrd archive is loaded in ram disk at boot by lilo or grub and contains driver modules and startup script that allow the linux kernel to load the mimimum module drivers to load the other modules.