Spread is the difference between the bid and ask prices of the same asset (currency, security, commodity, etc.).
Let's consider this concept in more detail on the example of securities. For example, an investor wants to buy shares of a notional company A and is willing to pay $550 for one security. At the same time there is a seller in the market who puts out securities of the same company, but sets a minimum price of $553 for one security. In this case, the spread is calculated using the formula: $553 - $550 = $3 (on the market this figure is measured in points).