Diamond clarity is the measure of a diamond's flaws, technically called
inclusions. Every diamond has some inclusions, even a diamond with a
flawless clarity grade -- but the inclusions in a flawless diamond are so small that they are undetectable at 10-power magnification by a trained diamond grader. All diamonds will have inclusions at higher magnifications, because you want a diamond created by nature rather than in a lab.
We usually recommend diamonds without inclusions or flaws that are visible to the naked eye. This avoids inadvertent negative feedback from friends and ensures the wonderful, lifelong enjoyment of your diamond.