The bid is the highest price a buyer will currently pay; the ask (or offer) is the lowest price a seller will accept. The gap between them is the spread - and crossing it is a real cost you pay every time you enter or exit a position.

Why the spread exists

Market makers quote both sides and earn the spread as compensation for providing liquidity and taking on risk. Buy at the ask and immediately sell at the bid, and you are down the spread before the market moves at all. On a tight, liquid product that is pennies; on a thin one, it is a real tax.

What widens it

Spreads widen with low liquidity, high volatility, and uncertainty. A calm midday SPY spread is razor-thin; the same spread during a news spike or in extended hours can blow out. Far-out-of-the-money and low-volume options carry the widest spreads of all - which is exactly where beginners get quietly fleeced.

You do not need the market to move to lose money. Cross a wide spread and you have already paid.

Why it decides where you trade

Tight spreads are a huge part of why serious short-term traders concentrate on liquid instruments like SPY. Deep, tight markets mean minimal slippage and clean fills. Using limit orders lets you control the price you pay relative to the spread - a small discipline that compounds across hundreds of trades.