Volatility measures how much and how sharply an investment's price moves, in both directions, over a period.
It is often used as shorthand for risk, but the two are not identical. Volatility is the size of the swings; risk, for most people, is the chance of not having the money they need when they need it. A volatile asset held for thirty years may be far less risky in that practical sense than cash losing value to inflation. What volatility does reliably predict is discomfort — and the real danger is that discomfort causing someone to sell at the bottom.