
It's difficult for me to explain (especially as I don't understand it myself), but at least he did give me some kind of approach to getting my head wrapped around it: Picture a flat world where all the people living there are two-dimensional. Then try to explain to them how you can bend their world and connect two distant points. It's the same thing with the Door, except it's a three-dimensional world being bent through the fourth dimension.
It seems that the topic has been covered quite well, too. You just have to know where to look for the info. I'll see if I can get a hold of some of those reference books and let you know what I find by next week.