the boot block
Think of the very first sticky note a new employee finds on an otherwise blank desk: it does not contain the whole job manual, just enough instructions to say 'go to this drawer, open that binder, start reading there.' When a computer powers on, its memory is empty and it has no idea how to load the huge operating system off the disk. The boot block is that first sticky note — a tiny, fixed-location program on storage whose only job is to start the chain that eventually loads the full OS.
Concretely, after the firmware (such as UEFI or the older BIOS) finishes its hardware checks, it reads a specific, well-known spot on the chosen storage device — traditionally the very first sector of the disk, the boot block (in the classic PC scheme called the master boot record, MBR) — and loads its small program into memory and runs it. This tiny boot program is deliberately simple, because it must fit in one sector. Its task is usually not to load the whole OS itself but to find and load a larger, smarter bootloader, which in turn locates the operating-system kernel on disk, loads it into memory, and hands control over to it. This step-by-step handoff, from firmware to boot block to bootloader to kernel, is called bootstrapping (the system 'pulls itself up by its bootstraps').
Why it matters: the boot block solves a genuine chicken-and-egg problem — you need a program to load the OS, but loading programs is normally the OS's job. The fixed-location, tiny boot block breaks the deadlock by being something the firmware always knows how to find and run with no OS present. Because it sits at a known fixed spot and runs with full privilege very early, it is also a classic target for attackers (boot-sector malware) and a key thing that secure boot tries to protect, by verifying that each stage in the chain is the genuine, signed code before running it.
You press the power button. Firmware tests the hardware, then reads sector 0 of the disk into memory and jumps to it. That tiny boot-block program loads the bootloader, which finds and loads the kernel, which finally takes over and starts the OS — each stage handing off to a bigger, smarter one.
A one-sector program that bootstraps the whole OS into life.
The boot block does not contain the operating system — it is only a tiny first link that loads the next stage. Because it runs so early with full privilege, it is a prime malware target, which is why secure boot exists to verify each stage.