Going "short," taking a "short" position, "shorting" or "short-selling" is entering a trade where you profit if the price of the asset goes down. This is the opposite of a long position where you profit when the asset increases in price.

The underlying mechanics are a bit bizarre, and vary depending on the type of trade.

In the securities markets, like stocks and bonds, one borrows the shares, sells them immediately, and aims to buy them at a lower price, in which they return them to the lender and pocket the difference.

With futures/options trading, the trader starts by purchasing the futures contract which is the right to sell the asset at a fixed price at some point in the future. If the price goes below that price, they can purchase the asset at that new price and sell it at the price set in the futures contract.

The main thing to remember is you enter a short trade if you believe the price will go down.