There's actually a new technology that can transmit your brainwaves wirelessly into a machine. It's the closet thing mankind has developed now to telepathy, and it's already on the market. It's being developed as both a cool gaming application called the Force Trainer, but there's also more practical applications. A company in Japan is using the same kind of technology to allow paralyzed people to control their wheelchairs using their mind!
That's an extreme example of something which can be as simple as a switch on a computer. A human-machine interface is just what it sounds like: something that allows a human to communicate with a machine, sending instructions or commands which are executed by the machine. One engineering site suggests that knobs and even levers are examples of a human machine interface, since they ultimately accomplish the same purpose: they allow a human to control a machine. Machines can perform calculations, mechanical tasks, and even major industrial processes - but they can't guess our intentions. In order for a machine to do what we want, there has to be some way to communicate instructions.
This has serious implications in the world of military technology. For example, the army is already deploying something called the Counter-Rocket Artillery Mortar System, an automated system to instantly shoot down any incoming rocket and artillery strikes. The machines have lightning-fast responses, which is crucial when an incoming missile can strike in just a few seconds. "Because of the very speed and nature of war, the computer has to respond without our authorization or interference," notes one technology author. "So we are a part of the loop in one sense, but we really aren't in another sense."
But even an automated machine has to be programmed by a human when it's set up for operation - and when it's finally taken down - through the use of a human-machine interface. There's already been a tragic example with an automatic weapons system in one country which, when programmed improperly, actually fired on its own troops before someone was able to intervene. It sounds like a science fiction story, but it's the world we're living in now. So the interface between a human and a machine must be clear and simple - or there could be unexpected and dangerous consequences.
But ironically, I found the single best explanation for human-machine interface as graffiti on the bathroom wall near my school's computer lab.
"Darn these machine, I wish they'd sell them. They don't do what I want. Only what I tell them."