/boot holds the files the bootloader needs to start the kernel: the kernel image itself, the initial RAM disk, and the bootloader configuration. Everything in /boot is read before the root filesystem is fully operational. Once the kernel is running and the system is up, nothing in /boot is accessed again until the next boot.

/boot/vmlinuz-6.1.0-28-amd64   compressed kernel image
/boot/initrd.img-6.1.0-28-amd64  initial RAM disk image
/boot/grub/grub.cfg             GRUB configuration (generated by update-grub)
/boot/grub/i386-pc/             GRUB stage-2 modules
/boot/System.map-6.1.0-28-amd64  kernel symbol table (used by crash tools)
/boot/config-6.1.0-28-amd64     kernel build configuration

/boot is almost always a separate partition formatted as ext4 or FAT32, depending on whether the system uses BIOS or UEFI. On UEFI systems the EFI System Partition (ESP) is mounted at /boot/efi or directly at /boot. Keeping /boot separate means the root filesystem can use LVM, LUKS encryption, or any filesystem GRUB does not natively support — as long as /boot itself is readable by the bootloader.

The initrd (initial RAM disk) is a small temporary root filesystem loaded into memory alongside the kernel. It contains just enough drivers and scripts to mount the real root filesystem, including drivers for encrypted volumes, network filesystems, or unusual storage controllers. Once the real root is mounted, the kernel discards the initrd and continues booting from disk.