Well, funny you should ask, because I'm trying to think where I learnt this from. I think I was actually taught it in school many decades ago. Each note and each coin of our currency has a a value. It's not the coin or note itself which is valuable, but they're all rather like IOU notes, in that they represent a true national value, which is the gold in the vaults of the Bank of England. If we have a billion pounds worth of gold in our vaults, then a billion pounds of currency can circulate. Hence the term "printing money", because you can print as much money as you like, but it has to be backed up by a true thing of value, such as the gold. I think the Fort Knox gold plays much the same role.
Thinking about it, perhaps one of the problems with the global economy is, that not all money is actually backed up by a thing of value, which it should be. It's just printed paper flying around, and it's now being called in. We're all wondering "who has the money because no one seems to have any but it's all going somewhere", is because much of the money in circulation didn't actually exist, except as numbers on bank printouts. Now, all those who are owed money, want it back, and there isn't enough of it to go around, because because the global accounts are bigger than the global amount of money actually in circulation.
