Bollinger's "simple" explanation for a Squeeze is the lowest volatility in the last 6 months. This indicator uses a default look-back period of 125 bars to determine the lowest BandWidth. When current BandWidth drops below the lowest BandWidth of the look-back period, the background of the bands turns red. Default look-back of 125 bars is ~6 months on daily charts.
The source, length, and standard deviation for the Bollinger Bands can all be adjusted. The look-back period for the Squeeze indicator can be adjusted as well.
The image shows my Bollinger Bands w/ Squeeze Alert indicator next to someone else's Bollinger Bandwidth w/ Squeeze Alert indicator to demonstrate how it appears on the chart.