That is a 'volume at price' indicator, showing the volume transacted at that price. So it comes out as a histogram and shows the 'value' areas, where the market found agreeable buying and selling. These areas tend to line up with support/resistance levels and it helps me to build a better picture of what is happening in the market.
If you're interested in more information about it have a google of 'market profile'. Which takes the concept further.
