Bollinger Bands %B or Percent Bandwidth (%B) is an indicator derived from the standard Bollinger Bands indicator. Bollinger Bands are a volatility indicator which creates a band of three lines which are plotted in relation to a security's price. The Middle Line is typically a 20 Day Simple Moving Average. The Upper and Lower Bands are typically 2 standard deviations above and below the SMA (Middle Line). What the %B indicator does is quantify or display where price is in relation to the bands. %B can be useful in identifying trends and trading signals.
Read more about Bollinger Bands %B
in TradingView wiki.