Biomatter conversion is a process through which the tissue of living organisms is transformed into fuel. The fuel produced by GAIA's terraforming machines is known in the world of Horizon Zero Dawn as Blaze.
The biomatter conversion process was first used by FAS in its Chariot line of "peacekeeper" robots, intended to eliminate the need for refueling and allowing the robots to stay independent indefinitely. The biomatter source could be anything, and although plant life was primarily used, living animals could be turned into fuel too.
When the Hartz-Timor swarm went rogue due to the Glitch, it followed its programming and began using any and all biomatter it could find to fuel the swarm and the creation of new robots. As more robots were made, more biomatter was required, and thus the swarm began consuming all life on Earth. Eventually, it succeeded in this directive, and Earth was rendered a barren rock, unable to support life as even the atmosphere had been stripped.
Biomatter conversion is performed by many acquisition type machines. The most basic herd machines, such as the Grazer and the Strider, were made for the purpose of gathering biomass and converting it into Blaze for use in fueling the Cauldrons and general power. It is unknown if other, non-herd machines (such as the Sawtooth) can also use this process for self-fueling, as they do not seem to consume any biomatter to this end.
The Corruptor and Deathbringer, being old remnants of the Faro Plague, presumably use biomatter conversion to fuel themselves as they did before, though being only of a limited number, they do not have as much of an impact on local biology as they once did.
Biomatter conversion is a key part of HADES' plans; due to GAIA self-destructing and its normal means of terraforming being out of reach, HADES intends to use a reactivation signal to resurrect the Faro Plague and use its indiscriminate biomatter consumption to fulfill HADES' primary function of destroying the biosphere. Near the end of the game, HADES begins to wake the swarm using one of MINERVA's towers, which then starts to consume the local plant life. Aloy stops the AI before the Plague can fully activate, and all the activated units shut down and the biomass conversion stops.