The init executable is responsible for the transition to userspace by starting essential drivers (such as disk drivers) and bootstrapping programs (e.g. another init system or user login service). It is launched as the last step of the startup procedure.
The executable is embedded in the kernel binary in order to alleviate the need for in-kernel drivers, requiring a kernel rebuild to apply changes and making compiling the kernel without it impossible (and pointless).
Further essential programs (including drivers) required by the init executable can be embedded within it or loaded using an embedded storage (disk or network) driver.
The kernel expects a statically linked ELF executable and starts it with PID 1 (PID 0 is a pseudo-ID used as the address space identifier (ASID) of kernel memory).
Example programs are provided in the examples/ directory.