It's an accepted and established value. Think of it as a baker offering to trade you x- amounts of baked product per month for something you have to offer, such as a certain number of eggs from your hens per month, or an actual chicken, or even a hog or lamb per year. Attach an agreed label to it such as a "Johnthebaker" Local Reserve Note (LRN) and use it as a standard so others can reference to it and trade them so that you can trade your neighbor for an LRN or two in exchange for a butchered turkey so you do not have to go to the bakery, and you have an established standard.
Extrapolate that to the Internet, with more people trading things and LRNs and that is essentially what Bitcoins are.
Rather like trading stocks, or if you work for a bank, trading mortgages.
...think of it as a baker offering to trade you x amounts of baked goods for something you have to offer...That's just it. There are no baked goods, hens, mortgages or anything else. What do I do? Just tell someone you can have 3 quadroogles for one bitcoin?
It seems to me that the big boys in getting those computer keys to sing have come up with something out of nothing and everyone nods their heads, says "wow" and goes along because they don't want to look dumb.