Initrd
From ArchWiki
What is an Initrd?
Initrd stands for "initial ramdisk". It is a gzip'ed image that is read by the kernel during bootup. The goal of the initrd is to bootstrap the system to the point where it can access the root filesystem. This usually means loading the necessary modules so the kernel can "see" things like IDE, SCSI, SATA, Firewire, or USB devices, any of which might contain the root filesystem.
Once the initrd has finished its business, control is passed to the init(1) process (as usual) and system startup continues as normal. For this reason, the initrd need only contain the modules necessary to access the root filesystem. It does not need to contain every module you would ever want to use. The majority of modules are still loaded with hwdetect, hwd, modprobe.conf, or rc.conf.
Arch Linux now uses an initramfs instead of an initrd in its boot process. While an initramfs does the same as an initrd, it is technically different. Therefore, the mkinitrd tool is now obsolete and has been replaced by mkinitcpio. If you still haven't changed your bootloader to the new image files, please do so immediately.