A more thorough explanation:

A short sale against the box is a type of short sale in which the seller already owns enough shares of a security to cover the sale, but still borrows shares to keep ownership a secret or because the owned shares are not easily accessible. The phrase "against the box" refers to the owned shares that are in safekeeping. This type of sale is less risky than an ordinary short sale because delivery can be made with either the owned or the borrowed shares.

For example, let's say John owns 100 shares of XYZ stock and wants to sell them short without revealing his ownership. He borrows another 100 shares of XYZ stock and sells them on the market. If the price of XYZ stock drops, John can buy back the 100 shares he borrowed at a lower price and make a profit. He can then choose to deliver either the owned shares or the borrowed shares to complete the sale.

This type of short sale is often used by investors who want to speculate on a stock's price without revealing their ownership or by those who own shares that are not easily accessible, such as shares held in a retirement account.