Volume is simply the number of shares that have been traded over a given period (eg. weeks, days or hours). In turn, the Volume Oscillator identifies volume trends by using a Fast Moving Average (typically a 28 day MA of volume) and a Slow Moving Average (typically 14 day MA of volume). Their difference is then plotted as a histogram.
Traders could use the Volume Oscillator to confirm price trends:
When the Fast Moving Average is higher than the Slow Moving Average, the Volume Oscillator will be positive. This could signal a strong price trend.
When the Slow Moving Average is higher than the Fast Moving Average, the Volume Oscillator will be negative. This could signal a weak price trend.