What is Grain of fact table?

According to Ralph Kimball, it is important to declare the grain of the fact table. Grain is the
smallest unit of occurrence of the business event in which the event is measured. In other words, grain is completing this sentence: “One row in the fact table corresponds to. . . .”


Ex: For a fact table storing product sales, the grain is each item sold—one row in the Product Sales fact table corresponds to each item sold