initrd image is the initial ram disk image
While the system getting booted the kernel image
will get loaded into the main memory after POST
to improve I/O performance.
this initrd image will contain the same version number
of kernel and if we face any error in booting regarding
this initrd we can reinstall it with 'mkinitrd' command

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initial RAM disk (initrd) is a temporary root file system 
that is mounted during system boot to support the two-state 
boot process. The initrd contains various executables and 
drivers that permit the real root file system to be 
mounted, after which the initrd RAM disk is unmounted and 
its memory freed. In many embedded Linux systems, the 
initrd is the final root file system

initrd image is a image file which has initial modules which
are loaded in to the kernel while booting.

initrd image is a image file which has initial modules which
are required to boot like scsi card drivers which are not 
available inbuilt in the kernal loaded in to the memory  
while booting.