On balance volume is a way of measuring whether so called smart money is accumulating or distributing a stock. It was originated by Joseph Granville and has come into popular usage on chart services.
Check out the OBV line above and below the price chart here. that is on balance volume telling you that the underlying interest in buying this stock at its level is very high, and has remained very high.
http://stockcharts.com/h-sc/ui?s=PG....