Realised volatility measures what a price actually did. Implied volatility is what the options market expects it to do, and the gap between the two is itself a tradable view.
Volatility is not the same as risk, though the two are routinely conflated. It describes movement in both directions, and a permanently impaired asset that stops moving at all has very low volatility and very high risk.
It clusters. Calm periods follow calm periods and violent days follow violent days, which is why the market can spend months barely moving and then deliver a year of range in a single week.