Intel ME is a coprocessor integrated in all post-2006 Intel boards, for which this Libreboot page has an excellent description. The main component of Intel ME is Intel AMT, and I suggest you to read this Wikipedia page for more information about it. In short, Intel ME is an irremovable environment with an obscure signed proprietary firmware, with full network and memory access, which poses a serious security threat. Even when disabled from the BIOS settings, Intel ME is active: the only way to be sure it is disabled is to remove its firmware from the flash chip.
Before Nehalem (ME version 6, 2008/2009) the ME firmware could be removed completely from the flash chip by setting a couple of bits inside the flash descriptor, without the need to reverse-engineer the ME firmware.
Starting from Nehalem the Intel ME firmware can't be removed anymore: without a valid firmware the PC shuts off forcefully after 30 minutes. This project is an attempt to remove as much code as possible from such firmware without falling into the 30 minutes recovery mode.
If you want to understand how me_cleaner works, you can read the "How does it work?" page.
For pre-Skylake firmware (ME version < 11) this tool removes almost everything, leaving only the two fundamental modules needed for the correct boot, ROMP and BUP. The code size is reduced from 1.5 MB (non-AMT firmware) or 5 MB (AMT firmware) to ~90 kB of compressed code.
Starting from Skylake (ME version >= 11) the ME subsystem and the firmware structure have changed, requiring substantial changes in me_cleaner. The fundamental modules required for the correct boot are now four (rbe, kernel, syslib and bup) and the minimum code size is ~300 kB of compressed code (from the 2 MB of the non-AMT firmware and the 7 MB of the AMT one).
This project is based on the work of the community; in particular I thank Igor Skochinsky, for the core information about Intel ME and its firmware structure, and Federico Amedeo Izzo, for its help during the study of Intel ME.