Folks, I am not technologically sophisticated, when I was young a transistor radio was a marvel. My parents bought me a transistor radio for my graduation from high school. There were still airplanes with two wings. (Hope that helps you understand why I asked the question: WHAT IS UNLOCKING ONE”S ELECTRONIC DEVICE?
In simple terms, modifying the device's software so that it can be used in different ways. Some vendors design products they sell to be "locked" to certain service providers - so one version of their phone only works with AT&T service. Changing any device you own per se shouldn't be illegal, although clearly what you do with the modified device is subject to regulation.
The current regulations are equivalent to a law forbidding modifications to a car. In other words if you want to go drag racing it would be illegal to put a Chevy motor in a Ford chassis. That's clearly ridiculous, since once you buy the car you can do what you want with it off the roads. There are legitimate regulations about how you can use the modified device of course, so you can't cruise down I-5 in your dragster.
Cell phone companies, and Apple want to control how phones can tablets can be modified since it helps them maintain market share and use marketing techniques based on subsidizing the cost of hardware to lock in customers. Having the government help them lock in customers is at the root of the dispute.