Liquidity = the ease at which you can turn an asset into cash.

A market with deep liquidity can absorb large buys or sells without moving much. A market with very little will fall apart at the first sign of one person's big move. Plenty of markets effectively have none.

It's all well and good having an impressive headline price for an alt, but when you look at an exchange and the buys waiting it might only take a sell of $100 worth to reduce the price by 50%. A liquid market will have enough buys to swallow that sell without budging.