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 green .
You can change to a histogram, area, line or any other style of chart in the settings of the indicator
The black dashed line in the middle is 0 aka if u see the indicator go there it means there are an equal amount of longs and shorts