NR7 means the narrowest range of the last seven bars; NR4 uses four. The logic is that volatility is mean-reverting: unusually quiet periods tend to be followed by unusually active ones.
Traders use narrow range bars as a signal to prepare rather than to act, setting orders above and below the bar and letting the market pick a direction.
Two cautions. Direction is not predicted, so a two-sided approach or a strong trend bias is needed. And the expansion may come with a fakeout first, taking out one side before running the other. Measuring range relative to atr rather than raw points makes the signal comparable across instruments and over time.
Related: inside-bar, wide-range-bar, atr