initrd is a method for loading a temporary file system into memory, during the process of booting the Linux kernel.
The initrd is mounted prior to the availability of the "real" root file system. In the case of embedded systems, the initrd is the file system image. See also initramfs, a newer similar facility.
Usually it will contain the minimal utilities needed for providing access to storage devices and filesystem.
For more information, check the article Linux initial RAM disk (initrd) overview.