Liquidity refers to the ease with which an investment can be bought or sold without disturbing its price. The more shares you can buy or sell at the current quote, the higher the liquidity. Of course, what's liquid to a person wishing to trade 100 shares could be highly illiquid to another with a 10,000-share order. A fund that trades 200,000 shares on average is more liquid than one that trades 20,000 shares. The size reflects demand and supply.
Bigger numbers indicate greater liquidity.
Size changes throughout the day so savvy investors track it closely when attempting to buy or sell shares. The size is available online if you have access to real time quotes, or through your broker. If you decide to buy or sell a large block of a thinly traded fund you may want to break your order up into several smaller ones and feed them in gradually to avoid putting excessive pressure on price, thereby minimizing market impact costs. The careful placement of limit orders can be a great way to minimize trading costs. Limit orders are instructions to buy or sell at a specific price or better.