The indicator plots the CFTC Commitments of Traders data.
The default setting is the Leveraged Funds numbers, which related to relatively fast money, which is where the fingerprints of the Composite Man are most obvious.
Red lines mean short positions, green lines are longs and the blue line shows the difference.
When the blue line is moving up, the Composite man is probably accumulating. When it is moving down he is probably selling off his stocks.
Remember that the direction of movement of the blue net line is more important than the absolute value, as there are perfectly good reasons to have a futures position to hedge other trading activity, but the overall trend is there to be seen.