Volume displays the volume of an instrument and is transmitted from the different exchanges. Volume is usually displayed as a histogram where each bar corresponds to a specified period of time.
Volume can provide clues into the underlying strength of a price trend. For example, if during a rally, volume levels are high, this would indicate strength in the rally. But if the rally continues at a slower pace, and with the same high volume levels, this may suggest a rally that is losing momentum.
Low volume levels usually occur during indecisive periods, such as market bottoms and consolidation periods, whereas high volume levels are usually associated with market tops and beginning of new trends.