The Buffett Indicator is the ratio of total the United States stock market valuation to GDP. It is said to be the best predictor of market corrections or crashes. 

As of May 6, 2021:

• Aggregate US Market Value: $51.8T
• Current Quarter Annualized GDP: $22.6T
• Buffett Indicator: $51.8T ÷ $22.6T = 229%

Historically, when the stock market went above GDP by 50-100% it crashed, going below GDP. 2021 is the year in the last 100 years when the ratio was over 2x the largest stock market in history.

In 2021, Elon Musk asked Cathie Wood, the famous tech-focused investor from Ark investments, what is her explanation for the high Buffet indicator (a ratio of US stock market/GDP). Cathie suggests a few reasons:

• while the indicator maintained its pattern over the last 100 years, in the late 1800s was much higher, suggesting tech infection points (telephone. electricity & automobile back then)
• the new tech is deflationary, making lowering the basket included in GDP. So denominator goes down while the numerator.
• as bonds yields ~1-3%, money is gonna move to other assets, like stocks.