The VIX is derived from the prices of S&P 500 options across strikes. A reading of eighteen implies an expected annualised move of eighteen per cent, which works out at roughly one per cent a day.
Below fifteen describes complacency, above thirty describes stress, and the extremes are rare: the VIX exceeded eighty in 2008 and again in March 2020. Because it spikes when equities fall, it is inversely correlated with the index by construction.
It cannot be bought directly. The tradable products hold futures, which in calm markets cost more than the spot index and decay as they converge, so a long volatility position held as a permanent hedge bleeds steadily between the events it is meant to protect against.