Often, an item comes in a variety of sizes and colors. A shirt, for example, might come in four different sizes and six different colors.
Each color and size combination is called a variation. Each variation, such as a medium, blue shirt, is an item that a customer can buy. For that reason, each variation has its own ASIN. For example, if a shirt came in four sizes and six colors, there would be 24 variations, each with a unique ASIN.