You think oil will get more expensive. But you don't need a barrel of oil: there's nowhere to store it, and selling it later isn't easy. This is where a contract with a broker helps. A broker is a middleman company you trade through; more about brokers two lessons from now. The contract says: “If the price goes up, the broker pays you the difference. If it goes down, you pay the difference.” This kind of contract is called a CFD, short for “contract for difference.” You don't have any oil, only a contract about its price.
An example. You opened a contract on 10 barrels of oil when a barrel cost $70. The price rose to $75. The difference is $5 per barrel, $50 in total. That's yours. If the price had fallen to $66, you would have paid $40.