Steve, it basically means that the ground plug is isolated from the conductor box (house ground) all the way to the breaker box. 10-3 Romex actually has 4 conductors. Hot, neutral, 2 grounds, one ground for electrical circuit, one ground for box. In theory it's to isolate any EMF or RMI noise that's being picked up by the house ground. Is it overkill? Probably. My electrician buddy says it is. However the PS Audio Power Ports I bought for him to install had this option, so I used it. The ground for the ground prong doesn't touch the metal plate. There isn't much difference money wise between regular 10-2 (hot,neutral, ground) and 10-3, so why not. Could I tell the difference between a dedicated circuit and a dedicated isolated circuit? Who knows? It only cost me $250 for the dedicated, isolated upgrade, and I can definitely hear the difference from before. However since I got the isolated ground at the same time as the dedicated line, I can't say for sure whether the isolation has anything to do with the new performance.
I hope this was helpful, I'm no electrician, but that's the best explanation I can come up with. I do KNOW that the 10-3 Romex he used had 4 conductors.