Textbook of Linux

/boot holds the files needed to start the kernel: the kernel binary itself (vmlinuz-*), the initramfs or initrd (initrd.img-*, initramfs-*.img), the bootloader configuration (grub/grub.cfg), and sometimes a System.map symbol table and a config-* file recording the kernel build options. On UEFI systems /boot/efi is typically mounted as the EFI System Partition, a small FAT filesystem holding bootloader EFI executables.

On systems with full-disk encryption, /boot is usually left unencrypted (or on an encrypted LUKS volume with a separate key) so that the bootloader and kernel can be read without unlocking the main root filesystem. Some modern setups go further and keep only the EFI partition unencrypted, loading the kernel directly from there via Unified Kernel Images.

Keep an eye on /boot's size. Each installed kernel consumes 50–200 MB between the vmlinuz and initramfs, and if /boot is a separate, small partition it can fill up after a few kernel updates. Debian's apt autoremove and Fedora's dnf autoremove will clean up old kernels, keeping only the two or three most recent. On a broken system, GRUB's menu still lets you boot an earlier kernel from /boot, which is often the quickest recovery from a bad update.