During boot process, the system will read the grub config to be able to load the kernel with it associated modules containing into initrd.

Initial RAM disk (initrd or Initramfs) is a RAM filesystem mounted at startup to initialize kernel modules.

You can read the content of initrd

# lsinitramfs /boot/initrd.img

Uncompress initrd

# mkdir /tmp/initrd
# cd /tmp/initrd
# cp /boot/initrd.img .
# unmkinitramfs initrd.img .