Liquidity

Assets with high liquidity can be bought and sold in large amounts while not effecting the market price. These assets have a lot of depth on their order books, which allow for purchases of thousands, or even millions of dollars worth of an asset without shifting the price.

Assets with low liquidity are very hard to buy or sell in large amounts. They usually have sparse order books, so trying to sell or buy any non-trivial amounts will cause significant changes to market price and market cap.