For example, if a trader is long in pork bellies, the trader is looking for the price of pork bellies to increase. But the price of bellies may not move in the direction the trader hoped. In fact the price of pork bellies may go against the trader's long position. Instead of going up as hoped, the price could decrease. If the price did go down and the trader decided to liquidate at a lower price, the trader would lose money.
ALL traders have losses, including the professionals.
Losses are unavoidable for the simple reason that futures trading is investing money in anticipation of future price moves. And prognosticatin', or whatever it was that Huckleberry Finn talked about with Tom Sawyer, can be a tad fuzzy. Some industry reports indicate that most traders are wrong most of the time. But the number of losing trades isn't the whole story. The money is.