Shorting is explained in full under short selling, which covers the mechanics, rules and controversies. This page explains the short form of the word, as traders use it: "a short", "to short" or "going short".
To short an asset is to position yourself to profit if its price falls. In the classic form, a trader borrows shares or coins, sells them at today's price and later buys them back to return to the lender, keeping the difference if the price has dropped. A "short" is the resulting position, and a trader holding one is "short" the asset. The opposite is a long position, a bet that the price will rise; the two are compared under long and short positions.
In crypto, most shorting does not involve borrowing coins directly. Traders instead open short positions in perpetual futures or other derivatives, often with leverage. Buying a put option is another way to bet on a fall with a capped loss.
Shorting carries particular risks. Because a price can in principle rise without limit, losses on a short can exceed the original stake, and leveraged shorts can be closed automatically by forced liquidation. When many traders are short and the price rises, their rush to buy back can push it up further, a short squeeze.