LOC = Line Out Converter. It's an electronic gizzmo that lets you tap into the speaker wires of an existing system and feed an amplifier's inputs. Many amps have one built in, they call it High Level Inputs.
You tap into the biggest speakers in the car, because those will have the most bass signal. Unfortunately many factory systems filter the deep bass out so you don't blow the stock speakers. That's why someone suggested tapping into the wires between the factory HU and amp - those will be "unfiltered".. but it's more complicated.