Bollinger Bands bracket a moving average (classically 20 periods) with bands two standard deviations wide, computed from the same window. Price at a band marks a statistically stretched move; band width itself measures volatility, and unusually narrow bands mark the compression that precedes expansion.
The width reading is often more useful than the touch reading: band width in the bottom quartile of its own history is a measurable definition of volatility contraction, one of the core criteria in breakout base screening. Touches, like RSI extremes, mark strength as often as reversal in trending tape, which is why they vote inside a composite rather than trigger anything alone.