What liquidity describes
Liquidity describes how readily an asset can be bought or sold in the desired size without causing a large change in price. It depends on available counterparties and conditions at the moment of execution. A market can be active overall while offering little size at a particular quote. That is why trading volume, the spread and depth are related observations rather than interchangeable measures of the ease of completing your own transaction.
A practical comparison
Imagine two markets showing the same best selling price. In one, enough quantity is available at that price for your entire order. In the other, only a small part is available, so the rest would need higher prices. The headline quote is identical, but the average cost can differ. Comparing depth and intended order size helps explain why an apparently attractive price may not be available for the whole transaction.
What can change quickly
Liquidity can weaken during news, market stress or a session transition. A limit order can set a price boundary, but it may remain unfilled; a market order can prioritize execution while accepting price uncertainty. Plan for both possibilities and avoid treating past conditions as a guarantee. For any actual product, the execution policy and contract specifications determine how orders are handled when quotes disappear, trading pauses or only part of the requested size is available.