What is JTAG in regards to Android?

I see the term JTAG used for reviving dead Android devices that otherwise don't respond to anything and aren't seen when USB connected. It seems that it's something done from the physical hardware side of the device (as opposed to some pure software based solution) and I've heard the term used elsewhere (in gaming consoles). I don't find much else about it, and I'd like to know specifically how it relates to Androids and similar devices. It seems to be a term in electronics or electrical engineering, both of which I am unfamiliar with. I'm wondering what it is and why it can be used on hard bricked devices.