Often called RVOL, this ratio displays the average volume of the stock over the previous 10 days divided by the average volume of the stock over the previous 91 days.The ratio helps understand how in demand the stock has been in past few days.
Suppose the 10 day average volume is 10,000 and the 90 day average volume is 4000, then relative volume is 10,000/4,000 = 2.5. RVOL above 2 is considered to be a signal of high demand for the stock and when a stock is in demand, price tends to moves up very quickly.