IV rank locates a stock's current implied volatility between its 52-week low and high on a 0-100 scale: (IV - low) / (high - low). IV percentile, often shown beside it, is the share of the year's days with lower IV. Both say whether premium is rich or thin for this particular stock.
The percentile form (counting the past year's readings below today's) is more robust than the min-max form, which one spike day can distort for a year. IV rank's classic failure mode is the aftermath of a volatility spike: rank stays high because the spike stretched the window, while the sellable premium has already deflated. Cross-checking rank against NVRP, which measures the premium itself, resolves the disagreement; the full treatment, with a worked example of rank and percentile disagreeing, is in IV rank vs IV percentile.