What they are
Bollinger Bands are three lines drawn around the price. John Bollinger introduced them in the 1980s. The middle line is the 20-day simple moving average. The upper and lower bands sit two standard deviations above and below it, so they widen when the price swings more and narrow when it calms down.
How traders read them
Most closes land inside the bands, so a close outside them marks an unusually large move against the last 20 days. That describes the size of the move, not a turning point: in a strong trend the price can ride along the upper band for weeks.
When the bands narrow to their tightest in months, traders call it a squeeze. It is a quiet stretch, and quiet stretches have often come before a larger move, in either direction.
Bollinger Bands on Breakaway
Every public stock page shows where the bands sit and whether the price is inside, above, or below them. The bands themselves are not scored. One of the 14 chart checks, the Bollinger and Keltner squeeze, reads how narrow the bands are against the Keltner Channels, and that check does count toward the Quality Score.