The construction is model free. Instead of backing a volatility out of Black-Scholes, the calculation takes the prices of a wide range of out-of-the-money puts and calls across two expirations, weights each by the inverse square of its strike, and combines them into a variance figure that is then annualised and interpolated to exactly 30 days.
Two consequences follow. First, VIX reflects the whole volatility-skew, not just at-the-money volatility, so heavy demand for far out-of-the-money puts lifts it even when nothing else changes. Second, VIX is a calculation, not an instrument — you cannot buy it, only derivatives on it.
Example: VIX at 18 means the option market is implying roughly an 18% annualised move over the next 30 days, or about 5.2% for the month. It does not forecast direction, and a reading of 18 is entirely compatible with a 10% crash.
Related: volatility-futures, vix-options, implied-volatility, skew-index