Volume is a very important indicator that can reveal a lot about the price trend of stock. Volume is simply the amount of shares bought or sold in a certain time period. For example if trader 1 buys 500 shares of stock A. Trader 2 buys 1000 shares of stock A while trader 3 sells 2000 shares of stock A. Then the total volume is then 500+1000+2000= 3,500. So the volume increase whether it is a buy or sell order. In simple words essentially volume is just a measure of how much buying/selling took place in a certain period of time. The video below explains what is stock volume.
Chuck Hughes is a master trader who explains in the video below how he uses volume to confirm the stock price trends.
In the above video, Chuck explains how he uses the On Balance Volume Line.
This is another good video on how to use volume in your stock trading!