If green is over the red it means that longs are currently dominant on BitFinex for BTC.
Same goes if red is over green just that shorts are dominant.
If the green is over red the green will show how many more longs there are than shorts and if you hover over the red you will see how many more shorts there are compared to longs
Vice versa if red is over...
Pretty much Self-explanatory :)
The columns (green and red) are difference in long and short positions during that period. Positive number suggests more number of long positions and vice-versa.
Works for all allowed crypto-pairs.
A similar indicator was published by another user, but sadly he closed the source code