From Barrons' Streetwise public on April 9, 2011 titled "What is Low Volume Telling Us?":

"Volume, is the weapon of the bull, historically, and is generally essential to push the market higher."

...all else being equal, brisker volume lends more credence to a market move.

...when volume begins to pick up meaningfully…it's often close to a cyclical [market] peak in need of a consolidation," or a sideways-to-down rest period.

A three-billion-share day in NYSE stocks, now considered uneventful, would have been cheered as a feeding frenzy just a few years ago.