Futures explained
When someone refers to "futures," they're really referring to futures contracts. A futures contract says a contract holder will buy the underlying asset on a certain date regardless of the asset's market price at that time. They agree to a price when they purchase the contract. The underlying asset could be a physical commodity like corn or oil or another financial instrument such as stocks.
Futures contracts use a standardized quantity for each underlying asset. Oil futures, for example, trade in contracts for 1,000 barrels. Corn, on the other hand, trades with contracts for 5,000 bushels, and each bushel is 56 pounds.
When you buy a futures contract, your broker won't require you to stake the entire value of the contract. Instead, you'll only have to hold a small percentage of the cash needed for the purchase, which is called an initial margin payment.
The price of the contract will fluctuate. If you, as the contract holder, are showing too big of a loss, your broker may require you to deposit more money.
Most commodity traders will close a position before expiration. Most people don't have the space to store thousands of barrels of oil or (literally) tons of corn.
When you sell a futures contract, you should receive enough funds to cover the margin loan, and, hopefully, have some left over as profit.
For example, if you bought an oil futures contract for $70, and the price goes up to $75, you'll make $5,000 ($5 x 1,000 barrels) when you sell. In the interim, you may only have to hold a few thousand dollars in your brokerage account, so the return on investment can be substantial.