The clarity of the diamond refers to the amount of flaws that are visible within the diamond. See chart below for more on Clarity.
Many inclusions and blemishes are too tiny to be seen by anyone other than a trained diamond grader. To the naked eye, a VS1 and an SI2 diamond may look exactly the same, but these diamonds are quite different in terms of overall quality. This is why expert and accurate assessment of clarity is extremely important.
Diamond Clarity | GIA Video
GIA created the 4Cs (Color, Clarity, Cut, and Carat Weight) over 50 years ago as a universal method for establishing the quality of any diamond, anywhere in the world. The 4Cs mean two very important things: diamond quality can be communicated in a universal language, and diamond-buying consumers can know exactly what they are about to purchase.