An average with bands a number of standard deviations above and below.
The bands widen when the market is volatile and squeeze together when it is calm. 'width' measures that squeeze.
Closing outside the upper band can mean strength (breakout) or an extreme (mean reversion). The band itself does not tell you which.
Worked out by the same code the builder uses, on stored daily candles. Move a setting and the line moves with it. This is what happened, not what will happen.
Gate: Bollinger width is rising, so volatility is picking up.
Touching the lower band in a downtrend is not a buy signal by itself.
No indicator predicts the market on its own. Build the rule, then test it on years of history with fees included. Try TradeXtool free for 14 days.