Volume is simply the number of shares that have been traded over a given period (eg. weeks, days or hours).
Investors can use Volume data to judge whether price trends are sustainable. If an uptrend is accompanied by rising volume, this could indicate that a growing number of bullish investors are buying the stock. The price trend could therefore be more sustainable. On the other hand, there are typically less active market participants during bearish phases. Therefore, if trading volume is falling or steady, it could signal that a market is becoming exhausted and a price reversal is around the corner.