This is far simpler than it sounds: it's the ease with which you can buy or sell an investment or asset. Cash is about as liquid as you can get. The house or flat you own (or don't own) is about as illiquid as you can get, taking months to sell.
For example, stocks which are easily tradeable on the NYSE and can be sold in a jiffy. On smaller markets, there can be fewer buyers to match up with sellers. This lack of liquidity can affect the selling process and the price.