Basically the TRIN measures the relative rate of volume that is flowing into advancing or declining stocks. So it tells you the general direction of price and rate of volume.
TRIN = (# of advancing issues/# of declining issues)/(volume of advancing issues/volume of declining issues)
The best advantage of this indicator lies in it's base. Most regular indicators are calculated from the price action... they represent some relation between the price of now and the price some time ago.
I use this indicator to give me a market direction in which I weight the signals more. So if the Trin is bullish then I doubled the positions on bullish market signals.