Volatility Skew is the difference in the Implied Volatility between out of the money calls and out of the money puts. Typically, Implied volatilities across different strikes exhibit what traders refer to as a "smile", i.e. "out of the money options" have slightly higher volatilities than "at the money" options.
However, occasionally the "smile" is "skewed", i.e. equally out of the money calls and puts differ in their Implied volatility. The skew thus represents the markets bias towards calls or puts.
We show this skew as the ratio of Call volatility to Put volatility. Therefore, a number greater than 100% means Calls are priced higher than Puts and vice versa.
In turn, this would show that the market has a positive bias towards the upside. On the other hand, if this number is less than 100%, then puts are being valued higher.