Let's say you are an airline and I am an oil company. You want to have a predictable cost for your fuel so you can plan, I want to charge whatever is the market price at the time. So a bank comes along and agrees, for a modest fee, that in a years time, you can buy fuel at a fixed price. In a year, they buy fuel from me and give it to you. If my price is lower, they pocket the difference as profit, but if it's higher, then they have to eat the cost and therefore make a loss (which is unlikely to be entirely covered by the fee you already paid them, but that's fine, because what you were really paying for was peace of mind).

People have been doing this with agricultural products for at least 3000 years.