Bollinger Bands measure volatility by placing trading bands around a moving average. These bands are charted two standard deviations away from the average, so as the average changes, the value of two standard deviations also changes. This value comprises the Bollinger Band Width, representing the expanding and contracting of the bands based on recent volatility.

During a period of rising price volatility, the distance between the two bands will widen (BB Width will increase). Conversely, during a period of low market volatility, the distance between the two bands will contract (BB Width will decrease).

There is a tendency for bands to alternate between expansion and contraction. When the bands are unusually far apart, that is often a sign that the current trend may be ending. When the distance between the two bands has narrowed too far, that is often a sign that a market may be about to initiate a new trend.

This chart from Dell shows Bollinger Band Width for a 20-day moving average using 4 standard deviations. It is easily seen how a high BB Width often indicates a slowing trend (red lines), and how a low BB Width often indicates a forming trend (green line).