This chart is here to illustrate for you how volume can help to clearly determine the trend. On each successive up move, volume has been higher than on the down move (retracements), shown even more clearly by the on balance volume line. (OBV) This is bullish. It takes volume to push up prices.

( Pll install traffic speed detection devices - a growth industry one suspects. The chart is indicating this company has some further good news to release.)

The OBV simply adds volume on up days (close higher than open) and deducts, from the cumulative total, volume on down days, (close lower than open).