Historical volatility, also called realized volatility, is the annualized standard deviation of a stock's actual daily returns over a lookback window, commonly 20 trading days (HV20). It measures movement that happened, where implied volatility prices movement expected.
The gap between implied and historical volatility is the volatility risk premium, the persistent tendency of options to price more movement than stocks deliver. Normalized as NVRP, that gap is a primary screen for whether premium selling is being paid. HV's main limit is that it is backward-looking: a scheduled event ahead makes trailing HV an understatement of what the near future holds, and one crash day inside the window inflates HV for the window's whole length.