Bollinger Bands wrap a moving average (usually the 20) in two bands set a number of standard deviations away (usually 2). Because standard deviation is a measure of volatility, the bands widen when the market gets volatile and contract when it calms — a self-adjusting envelope around price.
What the bands mean
The middle band is the mean (the 20 SMA); the outer bands are roughly the statistical edges of normal movement. By definition, price spends most of its time inside the bands, so a touch of the upper band means price is stretched high relative to recent volatility, and a touch of the lower band means stretched low. The bands measure 'how far from normal' price currently is.
The classic trap
The beginner mistake is identical to the oscillator trap: 'price touched the upper band, sell.' In a strong trend, price can 'walk the band' — riding the upper band higher for a long time. A band touch is not a reversal signal by itself. In a range, band touches do tend to mean-revert toward the middle; in a trend, they confirm strength. Regime first, always.
Better reads
More useful than fading touches: the middle band as dynamic support/resistance and trend reference (price above it = bullish bias), and the bandwidth itself as a volatility gauge (the squeeze, next lesson). Some traders also watch for a band 'walk' as trend confirmation and for price closing back inside after poking outside as a mean-reversion trigger — but only with the regime in mind.