Bid-ask spread
The gap between the highest price a buyer is currently willing to pay for an asset and the lowest price a seller will accept. It is a real cost: buying then selling back pays the whole of it.
Every quoted market has two prices rather than one. The bid is what someone will pay you right now, the ask, also called the offer, is what someone will sell to you right now, and in an orderly book the ask is the higher of the two. The difference between them is the spread, and it is what a market maker earns for standing ready to trade either side at any moment. The midpoint of the two, the mid, is the reference the pair of quotes straddles.
Spread width is the quickest read on liquidity a screen will give you. A heavily traded large company can quote a spread of a few hundredths of a percent, while a thinly traded small company, a corporate bond or a far out-of-the-money option can quote several percent. Spreads widen at the open, around news, and in falling markets, which is exactly when the most people want to trade.
The mistake is reading a commission-free trade as a free trade. The spread is charged whether a commission is or not. Buy at the ask and sell at the bid and you have paid the full spread across the round trip, so a position quoted at a 1 percent spread starts about 1 percent behind before the price has moved at all. The last price on a chart is a record of a completed trade, not an offer available to you, which is why a limit order exists: it is the standard way of refusing to cross the whole spread.