What is liquidity?
Liquidity, in simple terms is the ability to trade (buy and sell) an instrument and is considered good when there are many market participants interested in Buying and Selling the instrument at any volume. The easier it is to sell or buy the instrument at any given volume, the better the liquidity.
The more liquid an instrument, the tighter the spread usually, as the market participants compete to get the best prices. When there is low liquidity, the pricing is generally more erratic, and the spreads may be higher due to lower demand for the instrument.