Check the unit and coverage
A stock series may report shares traded, while a futures series may report contracts traded. A contract and a share are different units, so the numbers are not directly comparable. Quantity also differs from the money value of trading. A total value in currency answers a different question from a total number of shares or contracts.
Volume describes completed trading activity under the coverage of the data series. Before comparing two figures, check whether they refer to the same instrument, period and set of trading venues. A figure for one venue covers a different set of trades from a figure for several venues. Even within the same series, a bar for a complete day covers more time than a bar for a single hour.
Read the volume panel
A chart often places volume bars beneath the price chart. Each volume bar corresponds to an interval on the time axis. In the illustration, the tallest bar represents the largest quantity traded among the periods shown. Its height does not describe the size of a price range. The diagram intentionally contains only a volume series, so it does not show whether any of those periods closed higher or lower.
Common misunderstanding
Volume and the number of trades are different measures. One completed trade can involve many units, and several smaller trades can add up to the same total quantity. Higher volume means more quantity traded within the stated coverage. By itself, it does not determine a future price direction or guarantee that a price move will occur.