Volume indicator measures the power of supply and demand buying/selling in the market. It shows strength and weakness of any pair at any moment. small/short bars means weak trading volume, long bars means strong trading volume.
Similar to ADX, high volume suggests one of two cases : 1 - Trend is getting stronger, 2 - Possible trend reversal. Volume indicator by itself can't tell you exactly which case is it, so you would need to use other indicators to know that. like, Stochastics, Bollinger Bands and Moving averages.
For example, it can be used with Bollinger Bands to identify breakouts.