a small piece of machine code that lives on the start of a disk

The bootblock is a small portion of software code read from a specific area of boot device after control is handed to it by the BIOS (or similar structure depending of system architecture involved, eg: OBP on Sun SPARC systems). This small portion of code will then execute other code, usually read from a specific partition, that understands the file system structure in order for the boot process to proceed.