If it's worth saying, but not worth its own post (even in Discussion), then it goes here.
Notes for future OT posters:
1. Please add the 'open_thread' tag.
2. Check if there is an active Open Thread before posting a new one. (Immediately before; refresh the list-of-threads page before posting.)
3. Open Threads should be posted in Discussion, and not Main.
4. Open Threads should start on Monday, and end on Sunday.
A futures contract is one where you agree to buy a specific quantity of an asset today for a specific price, but you don't pay until a specified time in the future.
If you predict it will have future value $100, and it only costs $10 now, it's worth buying, hence there will be more demand, driving the price of the futures contract up. On the other hand, if it costs $100 today, but you expect it will cost $10 in the future, then the futures contract won't be worth as much, driving the price down.
We still expect the price to be about as good of an approximation of the future value as you can get (as long as the volume is high) - if you have a better prediction, you can make money off it! So the price of the future will reflect the best aggregate prediction of the future value of the asset. This is essentially the efficient-market hypothesis. This is the inspiration for idea futures, a.k.a prediction markets.
For NGDP futures, they would create contracts like this:
The prices of these contracts now would reflect the market's certainty that the future NGDP would be within that range.
Well, technically speaking the price of the future will reflect the capital-weighted opinions of the market participants. That is not necessarily the "best aggregate prediction" -- it could be, but there are no guarantees.