The stock market is a record of who wants to purchase or sell shares and at what price. This ledger is known as the limit order book, and it is what every trader strives to see since it reveals where there is supply and where there is demand.
Because of the competitive nature of the market, most of the liquidity that is visible is, in one way or another a bluff.
In general, the options market is seen as better informed than stock market (e.g. insiders trade via options, because of leverage + easier to mask positions).
Therefore, a heavy call/put skew is usually seen as a bullish sign, while the reverse is also true. Skew can be measured by comparing the at-the-money strike volatility of the 25delta calls and puts.
@MN Quants