It means far more shares changed hands than that name normally trades, which indicates broad participation rather than a few opportunistic orders. It says how many people acted — never which direction they were right about.
Every share sold is also bought, so volume alone carries no directional information. High volume means many participants transacted, not that selling outweighed buying.
What it measures is conviction and breadth: how much of the market took part in setting the day's price.
A million shares is enormous for a small company and trivial for a large one. Volume is therefore compared to that stock's own typical day, often as a multiple.
This is the same reason a percentage move means different things for different names — the baseline has to be the stock's own behaviour.
A large move on light volume can reflect a thin order book, where few orders were needed to shift the price. Such moves are more easily reversed.
The same move on heavy volume means many participants re-priced the stock together, which tends to be more durable.
Unusual volume with no visible explanation happens routinely — index rebalancing, a large holder adjusting, or option-related flows.
It is a prompt to look, not a conclusion. Volume describes activity; it does not identify a cause.
One question, no account: what did you come here for, and did you find it? A person reads these, and they shape what gets written next.
From StockMotive — the honest why behind every market move. Educational information only, not investment advice.