The VIX, or the CBOE Volatility Index, is a measure of the expected volatility of the U.S. stock market. It is calculated by the Chicago Board Options Exchange (CBOE) and is often referred to as the "fear index" because it tends to rise when market conditions are uncertain or volatile.

The VIX is based on the prices of options on the S&P 500 index, which is a broad measure of the performance of the U.S. stock market. Specifically, the VIX measures the implied volatility of these options, which is the expected amount of fluctuation in the price of the underlying asset (in this case, the S&P 500 index) over a given time period.

The VIX is expressed as an annualized percentage and is typically quoted in terms of points. A high VIX reading indicates that traders expect the stock market to be more volatile in the near future, while a low VIX reading suggests that traders expect relatively stable market conditions.

The VIX can be useful for investors as a way to gauge market sentiment and risk appetite. For example, if the VIX is high, it may indicate that investors are worried about market conditions and are seeking ways to protect their portfolios against potential losses. On the other hand, a low VIX may suggest that investors are feeling more confident about the market and are less concerned about potential volatility.