Volume is the most fundamental market indicator: how many shares changed hands during each bar. A big price move on high volume means lots of participants agree the move is significant. A big move on low volume is suspect. Fewer people are behind it, and it may not stick.

Volume tends to spike at important moments: breakouts, breakdowns, earnings announcements, and trend reversals. If a stock breaks above resistance on 3x its average daily volume, that breakout is much more likely to hold than one on normal volume. Conversely, if volume dries up during a rally, the rally may be running out of buyers.

The volume bar chart at the bottom of a price chart is colored to match the candle above it (green for up, red for down). This makes it easy to see at a glance whether buying or selling pressure is dominant. Volume is not a directional indicator on its own. It's a confirmation tool. It tells you whether to trust what the price is doing.