The basic form of the bid-ask spread is the difference between the highest price a buyer is willing to pay for an asset (the bid price) and the lowest price a seller is willing to accept for the same asset (the ask price). The bid-ask spread represents the cost of trading and is typically expressed in terms of a percentage or a dollar amount. The size of the bid-ask spread can vary depending on the liquidity of the asset, market conditions, and other factors.