Specifically, there are several quantitative measures of liquidity, and they're generally correlated. In particular, the average bid-ask spread (the time-weighted or trade-volume-weighted average between the lowest asking price from local BTC sellers and highest bid from local BTC buyers) and the average daily trade volume (add up the number of BTC exchanged to or from local currency in an average day). Low liquidity generally means wide spreads and low trade volumes. As a result, if you want to convert BTC into local currency in the morning to buy chickens in the market and convert your chicken soup earnings back into BTC in the afternoon, you're going to either buy high and sell low crossing the bid-ask spread each time, or you're going to wait a long time for someone else to cross the spread and fill your orders. Of course, you can offer price improvement, allowing a whole range of price-time tradeoffs, but time is going to be expensive all along the tradeoff curve.