What volume does and doesn't tell you
Volume counts transactions. It does not count buyers, and the difference between those two statements undoes most of what gets said about it.
Volume is the number of contracts or shares that changed hands in a bar. It is the only common chart input that is not derived from price, which makes it genuinely additional information.
The thing that gets said wrong
Every transaction has a buyer and a seller, in equal size. There is no such thing as more buying than selling. When people say “heavy buying,” they mean transactions occurred nearer the offer than the bid — which is an inference from where trades printed, not a count.
Platforms that split volume into buy and sell are applying a rule, usually comparing the trade price to the prevailing bid and offer. It is a reasonable estimate. It is not a measurement, and different platforms produce different splits from the same data.
What volume genuinely indicates
Participation. A large move on unusually heavy volume involved many participants. The same move on thin volume involved few. That is a real distinction, and it is about how many people were involved rather than what they concluded.
Unusual conditions. Volume far outside its normal range for that instrument at that time of day says something out of the ordinary is happening. It does not say what.
Compare to the right baseline
Raw volume is nearly useless because it varies enormously by time of day. Most markets show a pronounced U-shape — heavy at the open, thinning through midday, picking up into the close. A bar with twice the volume of the one before it may simply be nearer the open.
What is worth reading is relative volume: this bar against the same bar-of-day over recent sessions. That controls for the pattern and leaves the part that is actually unusual.
The equities problem
US equity trading is split across many exchanges and alternative venues, and a meaningful share executes away from the public exchanges. What your platform reports depends on which feed you pay for. Two traders can see materially different volume on the same stock at the same moment, and neither is wrong.
Futures volume is centralised at the exchange, so it is consistent between platforms and between traders. This is a concrete reason volume analysis rests on firmer ground in futures than in individual stocks, and a reason not to carry conclusions from one to the other.
What it cannot tell you
Direction, because every trade has both sides. Who transacted, or why. Whether the activity was a fund establishing a position, an index rebalance, or a hedge against something you cannot see. The bar records that a lot changed hands, and the reasons are outside what any chart contains.