What it means
Stocks are shorted by borrowing shares. Futures, forex, CFDs and perps are simply sold. Short losses have no ceiling, so stops matter even more.
Example
Short 10 shares at $120 and cover at $112: profit $80 before costs.
In practice
Shorting lets a trader profit from falling prices and is as normal a part of trading as buying. Traders look for shorts when the market is making lower highs and lower lows, ideally selling a rally into resistance.
In futures, forex and crypto derivatives a short is just a sell order. In stocks the shares have to be borrowed through the broker, which can cost a fee and is not always possible.
What to watch for
Markets tend to fall faster than they rise, but rallies against shorts can be violent, and a stock can gap up on news with no chance to exit. Always place the stop above the level that would prove the idea wrong.