Definition

Short selling is a trading strategy in which a trader aims to profit from a decline in a security's price by borrowing shares and selling them, hoping the stock price will then fall, enabling them to purchase the shares back for less money.

Source

In the simple form shorting the stock market is tantamount to betting against it.

I'm going to try to be as unbiased as possible in this opinion piece.

By now everyone has had their chance to rage against the openly corrupt congresspeople who bought and sold stocks based on their insider knowledge of various laws being proposed. And you should be outraged at this corruption.