High volume trading is trading in which an unusually large number of shares or contracts changes hands. The phrase is used in two ways. For a single security it describes a period in which trading volume is well above normal, usually judged against the average daily trading volume. For a firm it describes a business model that depends on very large turnover and a small margin on each trade, as in market making and high-frequency trading.
Heavy volume in a security generally means better liquidity and a narrower bid-ask spread, so large orders can be filled with less price movement. Volume spikes tend to accompany earnings releases, index changes and other news, and technical traders read a price move on high volume as more convincing than the same move on thin volume. Volume alone does not show direction, since every trade has both a buyer and a seller.