VSA is volume spread analysis and is the study of volumes and the relation between them and candle stick spread and wicks. You can tell a lot using volumes, though most people don't understand how to use them, yet I see so many people with them on their charts, and think because there is a pin bar and spike in volume, that means it will work. Not true, there is so much more, volume divergance, no supply/no demand candles, pump and dump (which creates huge spikes in volume).
What you need to understand is, volume is not the actual quantity traded, but the number of transactions that took place in the given candle. When trading VSA there are a few things you need to understand, and some main principles, there is accumulation, markup, distribution, re-accumulation, markdown... and so on. I wouldn't recomend using VSA though, it is hard to master, plus, you can tell everything from candle sticks and the patterns they make without using volumes. It just takes sometime to learn, but is possible.
Hope all that makes sence