Well you can explain bitcoins with reference to gold or diamond or any valuable stones, like how we buy such valuable stones at a certain price and later trade as per the fluctuation. If prices fall we wait or ay be cash out if necessary, and if prices rise we gain profit by selling/en-cashing them or may be keeping it for future and trading when prices go up higher. Bitcoins arent very difficult to understand, its just that children or for that matter even elderly's dont come across bitcoins or cryptocurrency in general in the first place. Hope that helps.