OBV is a very popular indicator developed by Joseph Granville and is a running total of volume that reflects accumulation or distribution. Each day's volume is assigned a positive or negative value depending on whether prices closed higher or lower that day and a running total is kept by adding or subtracting the volume depending on the direction for the day. The direction of the OBV line is the thing to watch not the actual level.
This indicator will often confirm underlying strength or weakness of a price trend and will often signal a top by declining while the stock is still rising, indicating big money is leaving the stock. Conversely, a rising OBV in the face of a declining price trend or an OBV that refuses to confirm a new stock reaction low could indicate smart money is moving in and a bottom may be at hand.