on my amp, there is something called a high input plug, what is it for?

Answer 1

its for an amplified signal as opposed to a low level signal which would be line in. a turntable would be a high input for example, probably some guitars and such.

Answer 2

normally you would use rca's to send a signal to the amp, but this allows for you to just tap onto the speaker wires and use speaker wire instead of rca's. this is good if you have a factory radio and you don't want to use a line output converter.