Liquidity is a measure of how readily an asset may be converted into cash or another asset. You may have the most priceless, rare old book in your bag, but if you're alone on a remote island, finding a buyer will be impossible.

On the other hand, if you want to purchase $100 USD of BTC on Binance's BTC/USDT pair, you may do it practically instantaneously and without any price effect. This is why, when it comes to financial assets, liquidity is critical.