As we talk more of the virtues of galvanic isolation in digital audio, perhaps it's of value to take a moment out of our busy day to understand just what that means.
The term galvanic refers to a galvanic cell, named after its inventor, Luigi Galvani. The Galvanic Cell is technically described with a mumbo jumbo of words: "an electrochemical cell that derives electrical energy from spontaneous redox reactions taking place within the cell."
More simply stated, it's a battery.
This can be confusing because when we refer to galvanic isolation we're not talking of batteries, but rather we're referring to electrical isolation. A battery operated flashlight can be said to be electrically isolated, but so too could we suggest a battery operated DAC is electrically isolated, yet we'd be incorrect unless that DAC's inputs and outputs are connected via optical cables.
And it is to this last point that we get to the heart of the matter.
To be galvanically isolated there can be neither physical connection nor direct electrical current involved. Even our old friend ground must not touch. In this way, whatever happens on one side theoretically shouldn't be mirrored on the other side of the circuit.
The methods of achieving such isolation are to connect via non-physical means such as magnetic fields or light—hence the use of isolation transformers and opto couplers.
None of this is easy to digest, but at least we can try and gain a little understanding.