中文：

卖空是指投资者在手中没有某只股票的情况下，以当前市场价格卖出该股票，并在未来以较低价格重新买入，赚取差价。简单来说，卖空就是投资者预测某只股票价格会下跌，从而以当前价格卖出该股票，等价格下跌后再以更低的价格买入。

卖空通常被视为一种投机行为，因为投资者必须承担市场风险和不确定性。如果市场走势与投资者预测相反，投资者将面临巨大的损失。因此，卖空需要投资者具备较高的风险承受能力和投资经验。

卖空在证券市场和期货市场中扮演着重要角色。在证券市场中，卖空者可以通过卖出股票获得现金，并使用这些现金购买其他股票或资产。在期货市场中，卖空者可以通过卖出期货合约来对冲未来可能出现的风险。

总的来说，卖空是一种高风险、高收益的投资策略，需要投资者具备较高的投资技能和风险承受能力。

英文：

Selling short is an investment strategy in which investors sell a stock they do not currently own at the current market price and then repurchase it at a lower price in the future to profit from the price difference. In simple terms, selling short is when investors predict that a stock's price will decrease and sell the stock at the current price. If the price decreases as predicted, investors can then buy the stock back at a lower price.

Selling short is typically considered a speculative practice because investors must bear market risks and uncertainties. If market trends move against investors' predictions, investors can face significant losses. Therefore, selling short requires investors to have high risk tolerance and investment experience.

Selling short plays an important role in securities markets and futures markets. In securities markets, sellers can obtain cash by selling stocks and use this cash to purchase other stocks or assets. In futures markets, sellers can sell futures contracts to hedge against future risks that may arise.

Overall, selling short is a high-risk, high-return investment strategy that requires investors to have high investment skills and risk tolerance.