Liquidity is the ability to convert an asset into cash. Generally speaking, an enormous 30,000 square foot mansion in Idaho is not very liquid – there just aren’t that many buyers so it might take years to sell. But a small home in Palo Alto is almost always liquid – at the right price – as there are always tons of buyers of a 2,800 square foot ranch home. Liquidity applies to stocks as well – Microsoft shares trade in the tens of millions per day; other small caps have days where only a few thousand shares trade. So if you own 100,000 shares of that small cap and need to get out, you’ll pay a dear price as the stock craters on your volume trying to get liquid in a security that ain’t.