The Linux kernel may boot using an "initramfs", an archive of files
that it unpacks into an in-memory file system at the beginning of the
boot process. This archive must contain its own init program which
the kernel will run as the first user process. In a Debian system,
the initramfs is responsible for:
* Loading essential driver modules
* Setting up a network connection, if required
* Setting up layered storage devices, if required
* Resuming from hibernation (suspend-to-disk), if possible
* Checking and mounting the root and /usr file systems
* Handing over to the main init system
initramfs-tools is designed to be extensible, and many other packages
provide these and additional features in the initramfs.
On x86 systems, the initramfs can also contain CPU microcode updates
that the kernel will apply at a very early stage. These are provided
by the (non-free) intel-microcode and amd64-microcode packages.
Debian's official kernel packages require the use of an initramfs,
since drivers and filesystem code are built as modules that need to be
loaded before the file systems can be mounted. Custom kernel packages
that have this code built-in may still need an initramfs to set up
layered storage devices or to provide microcode updates.
If initramfs-tools is installed, it will generate an initramfs
automatically whenever a kernel is installed or upgraded (and remove
it when the kernel is removed). If only initramfs-tools-core is
installed, you can run mkinitramfs to generate an initramfs.
The initramfs-tools(7) manual page documents the supported parameters
and how to extend initramfs-tools. There are additional manual pages
for the various commands and configuration files.