This is a little blurb that I use to help people grasp the underlying idea of currency

If I have $1, but I am the only person in the world, does that dollar have any value? No, you minus well burn it for warmth. Now, say there are 2 people in the world. Me and Chris. If I have $1, and Chris will accept that dollar in exchange for a slice of bread. Then, that dollar becomes valuable. So in essence "money" is the faith that the collective majority of people have in it.

Hope that helps.