At the lowest common denominator that is what volume does tell you. What makes volume more meaningful is its change from day to day or week to week. If the market is making a move higher that means there are more buyers in the market than sellers. On a move lower there are more sellers than buyers. That is rather obvious. To sustain a move higher, however, there would need to be more buyers coming into the market to continue to buy stocks. Otherwise a move would fizzle. We saw that several times during the downtrend with a few insitutions starting to buy that spiked up volume only to see the rally stall as no other buyers came into the market. That is why we look for another solid upside session on big volume a week or so later to confirm the first rally attempt. That means the buyers are coming into the market and overpowering the sellers. The volume is their footprint.
In general, as long as you are seeing more sessions where stocks rise on greater volume as opposed to falling on greater volume there are more buyers in the market than sellers. There can still be down sessions on rising volume, but as with most aspects of the market, the trend is the key. That is why when looking at individual stock patterns we look at the volume to see if the trend in accumulation is stronger than any distribution trend. Thus volume gives you good insight as to whether there is ongoing accumulation of stocks in the market and it can be used for individual stocks as well. If the big money is buying into your stock that gives it more staying power for longer term moves.