TL;DR Short selling means borrowing shares, selling them immediately, and hoping to buy them back cheaper later — profiting from a fall instead of a rise.
The plain-English version
Normal investing: buy low, sell high, in that order. Short selling runs the same trade backwards: sell high first, buy low later.
Concretely: you borrow 10 shares of a $50 stock from your broker and sell them for $500. The stock drops to $30. You buy 10 shares back for $300, return them to the broker, and keep the $200 difference (minus borrowing fees).
You sold something you didn't own, bought it back cheaper, and pocketed the gap. Completely legal, happening constantly, and one of the oldest trades in finance.
The catch — and it's a big one
Buy a stock for $500 and the worst case is losing $500. Short a stock for $500 and there is no worst case floor — because there's no ceiling on a stock price. If your $50 short runs to $200, you owe $2,000 worth of shares against the $500 you collected. Rises further, owe more. This is what "unlimited downside" means, and it's why shorts get forcibly squeezed (see: short squeeze).
Add the running costs: you pay interest to borrow shares (hard-to-borrow stocks can cost absurd annual rates), and you owe any dividends the stock pays while you're short. Time literally bills you.
Why shorts exist (and aren't villains)
Short sellers get cast as the bad guys, but they're often the market's fraud-detection layer — some of history's biggest frauds (Enron, Wirecard) were flagged by short sellers years before regulators moved. Shorting also lets market makers hedge, which keeps everyday trading liquid.
The common mistake
"This stock is garbage, so I'll short it." A stock being overvalued and a stock going down are different events on different timelines. Garbage can double before it dies — and your short can die first. The market can stay irrational longer than you can stay solvent.
Educational only — not investment advice. Shorting is a professional's tool with an amateur-shaped trap door.