The VIX derives an annualized expectation of S&P 500 volatility over roughly the next 30 days from option prices. It measures the expected size of moves, not the direction of the stock market.
History and development
VIX was introduced in 1993 using S&P 100 options. In 2003, its methodology changed to S&P 500 options, establishing the basis of the modern broader-market volatility measure. VIX futures followed in 2004 and options in 2006; those tradable products are distinct from the index itself.
References: Cboe: VIX history
Comparing charts on this board
VIX describes expected volatility implied by options, not stock-market direction. Beside the S&P 500, it lets you observe whether volatility expectations changed around an equity move. A rising VIX does not guarantee falling stocks, and a low VIX does not guarantee safety. Volatility includes the potential size of upward as well as downward moves.
Distinguish the VIX level from its daily percentage change. The chart's day-over-day change in VIX is not a forecast of the S&P 500's next return. VIX itself is also not a stock that can be held directly; related futures and ETFs have their own mechanics. Use the pairing to examine changing volatility expectations alongside prices.
Related charts: S&P 500 · NASDAQ 100
Official references: VIX overview