Liquidity is how easily you can buy or sell an asset without significantly moving its price. A liquid market has lots of buyers and sellers standing ready at every level; an illiquid one has gaps where a single order can jolt the price. It quietly governs your spreads, your fills, and your slippage.

Why it decides your fills

In a deep, liquid market, you can enter and exit large size at prices right next to the last trade. In a thin one, your own order eats through the available quotes and fills at progressively worse prices - the definition of slippage. The difference between a clean fill and a painful one is almost always liquidity, not luck.

What dries it up

Liquidity is not constant. It thins out overnight and in extended hours, evaporates during news shocks, and is always thinner in far-out-of-the-money options and low-volume names. The moment you most want to get out - a fast, scary move - is often the moment liquidity is worst, which is exactly why stops can slip.

Liquidity is the water you swim in. You only notice it when it is gone - usually mid-trade.

Why it favors SPY

The single biggest reason serious short-term traders concentrate on SPY is liquidity: it is the most heavily traded security in the world, with deep options markets and razor-tight spreads at nearly every strike. That depth means dependable fills and minimal slippage - the unglamorous foundation that makes a fast, rules-based approach actually executable.