Volume is the count of units traded inside a bar. It is the only piece of information on a typical chart that is not a transformation of price itself — every indicator you have ever seen is just price re-arranged, but volume is independent evidence of how much real activity backed each move.
Confirmation
When price breaks out of a range and volume surges with it, that is confirmation: a lot of money moved on that decision, and large flows are harder to reverse than small ones. When the same breakout happens on light volume, the breakout is suspect — it may have come from a handful of orders, easily faded by anyone willing to push back.
Divergence
The interesting case is Divergence — when price keeps moving but volume falls off. New highs on shrinking volume mean the trend is running on momentum, not new participation. That is often (not always) the late innings of a move, because the buyers who were going to buy already have.
What volume cannot tell you
Volume is conviction, not direction. A high-volume red candle and a high-volume green candle both reflect a lot of activity; the candle says which side won. Volume also lives in absolute units, so a 'big' day on one asset is a sleepy day on another — always compare a bar's volume to a rolling average of its own recent history, not to some universal threshold.
Reflexión
Pull up a strong rally on a chart you know well. Was volume rising into the move, flat through it, or fading? What does that suggest about who was driving it?