To short, your broker borrows shares from another account and sells them for you. You owe those shares back. If the price falls you buy them back cheaper and keep the difference; if it rises you buy them back at a loss.
Shorting has costs and risks that buying does not: borrow fees, the possibility the lender recalls the shares, and theoretically unlimited loss because a stock can rise without bound. A crowded short is what fuels a short-squeeze.
Example: you short 100 shares at $50. The stock drops to $40 and you cover: profit $1,000 minus borrow fees. If it instead rises to $70, you are down $2,000 and the loss keeps growing.
Related: short-interest, short-squeeze, margin, bear