can someone please explain volume?!!

Many good traders talk about how good of an indicator volume is. Despite looking and looking for an explanation I can't find anything that says how to incorporate it into a trading strategy and how to use it to determine if the trend is bullish or bearish. From all I can see if there is a big green candle then there is a big green volume bar, and if there is a big red candle then there is a big red volume bar. I can see many Instances where the trend goes way up but there are many small green volume bars and some big red ones mixed in. From what I can see the volume is only proportional to the size of the candlestick and nothing more. Id really like to understand this better, could someone please explain? thank you.