Volume is how many coins changed hands during a candle. It is the fuel behind a move. Price tells you what happened, and volume tells you how much conviction was behind it.
A move on high volume means many traders agreed and committed real money. A move on low volume means few people cared, so it is easier to reverse. The same green candle means very different things depending on the volume under it.
The most useful signal is a breakout with rising volume. If price pushes through a level and volume jumps, the move has real fuel. If price pushes through on thin volume, be careful, because moves like that often fade.
High volume confirms a move. Low volume warns that it may not stick. Always read price and volume together, never price alone.
Tip. A rally that keeps climbing on shrinking volume is running out of fuel. Fewer buyers are chasing it, even if the candles are still green.