What is HM?

Given the ambiguity of "HM", without further context, it's impossible to provide a specific technical explanation. "HM" could refer to a variety of technologies, companies, or acronyms. For example, it could stand for Hyper-Threading (a type of CPU technology), Head Mounted display (used in VR/AR), a Handheld Module, or it might even be an abbreviation for a company name. More context is needed to determine the intended meaning.