The gap between the best buy price and the best sell price; a hidden cost paid on every round trip.
The bid-ask spread. Buy orders sit below, sell orders above, and the gap between the best bid (50.01) and best ask (50.04) is the spread you pay to cross. Bar length shows the size resting at each price.
The spread is the ask minus the bid. It is how market-makers get paid for providing liquidity, and it is the first cost you pay on any trade, before commissions or slippage.
Wide spreads mean thin markets or high uncertainty; tight spreads mean many buyers and sellers close together. Spreads widen around news, at the open, and in extended-hours sessions.
Example: a stock trades at $10.00 bid / $10.05 ask. Buying and immediately selling 1,000 shares costs $50 in spread alone (0.5% of the position). A stock at $100.00 / $100.01 costs $10 for the same 1,000 shares, or 0.01%.