The bid-ask spread is the difference between the highest price a buyer is willing to pay (bid) and the lowest price a seller will accept (ask) for an ETF at any moment during the trading day. When you buy, you pay the ask price; when you sell, you receive the bid price. The spread is an implicit transaction cost. Highly liquid ETFs such as SPY typically have spreads of just one cent or less; smaller, less-traded ETFs may have wider spreads.