In the stock market, short sellers are often people who are betting against the price of a stock. They hope the stock will go down. If it goes up, they are wrong, and they lose money. If it goes up a lot, they will get margin calls; they will have to put up more money with their brokers to collateralize their risk. If it goes up a whole lot, they will have to cut their losses by buying back the stock, which will cause the stock to go up more, which will lead more short sellers to capitulate and buy back stock, etc. This is often called a “short squeeze.” When it happens, people who bet against the stock — short sellers — are sad and lose money, and people who bet on the stock — long owners — are happy and make money.
In commodities markets, short sellers are often people who produce the commodity. If you are an oil company, your future income will depend on the future price of oil. In order to make sensible budgeting decisions about how much to spend on drilling oil, you might want to lock in that future price. So you might sell oil futures today to guarantee you a price in a few months. Or you might not; you might be bullish on oil and want full unhedged exposure. Or you might hedge part of your production; if you plan to produce 1 million barrels you might only sell 500,000 barrels of futures. Or, since you are trading oil futures anyway and have some expertise in oil markets, you might end up net short, selling more oil than you plan to produce as a bet that prices will go down. But most likely you are in the oil business because you are hoping to make money drilling oil, and you are in some broad economic sense “long oil.” (You might hedge 50% or 100% or 150% of this year’s production, but you won’t hedge 100% of all of your future production.) If oil prices go down you will be sad; you will make money on your futures contracts, but that will only partly mitigate your sadness.