To long or short an asset, in general terms, is to place a bet on whether its price will go up or down in the future. Unlike trading between assets, where a "good trade" necessitates taking into account the changes in value of two tokens versus a unit of account, longing and shorting allows you to focus purely on the price future of one. The mechanism of placing a long or a short trade entails borrowing and thus acquiring debt or "leverage." This gives the practice the name leverage trading. The asset borrowed (either the unit of account or the asset to be shorted) is used to purchase more of the collateral used on the assumption that it will have a greater value in the future and allow for the debt to be paid off while leaving a portion (the profit) that can be retained.