Every time stocks shake, something appears. It's called the VIX, known as the fear index, but surprisingly many people don't really know what it is. It's the most intuitive tool for reading market sentiment, after all.

The reason it's nicknamed the fear index is simple. The VIX is a volatility index calculated by the Chicago Options Exchange in the U.S., and it quantifies the implied volatility of S&P 500 options. In other words, it reflects how much investors expect the market to fluctuate in the future, expressed as a number. If the VIX is 20, it indicates that investors expect the S&P 500 to move within a plus or minus 20% range over the next year.

When the market is stable, the VIX stays low, but it surges when anxiety spreads. This is because investors buy options or hedge their positions out of fear. So, a high fear index means the market is in a state of extreme anxiety.

Generally, a VIX of 15–20 is considered normal, 20–30 is a caution zone, and above 30 signals danger. Over 40 indicates an extreme state of fear, and during the early COVID-19 pandemic in 2020, it soared into the 80s. Conversely, falling below 12 can suggest that investors are overly optimistic, which might be a sign that a correction is coming.

An interesting point is that the stock market often rebounds 6 to 12 months after a sharp spike in the VIX. Extreme fear can actually be a buying opportunity at low prices. Financial experts also use this to predict market bottoms.

There are a few precautions when using the VIX for investment. First, you can't directly invest in the VIX itself—only through futures or related ETFs. Second, these products are mostly leveraged, so rollover costs can be high, and losses can be significant. Third, the fear index doesn't predict market direction; it only shows volatility, so it doesn't tell you whether stock prices will go up or down.

For more accurate judgment, you should look at other indicators like the S&P 500's trend, the call/put ratio in the options market, or the CBOE skew index. Comparing it with indices like CNN's Fear & Greed Index can also be helpful.

Ultimately, volatility is both a crisis and an opportunity. Reading the moments when the fear index surges and responding with proper timing is key to success, but always remember to stay cautious.