The clarity of a diamond is refering to the amount of imperfection, or not, within or externally on a stone. The less imperfections the better the stone.
GIA clarity’s are listed as follows:
IF. Internally Flawless
VVS1, VVS2. Very, Very Slighly included
VS1, VS1. Very Slighly included
S1, S1. Slighlty Included.
I1, I2, I3. Included.
All of the above are to a X10 loup magnification.
For more detailed information please go to the official GIA website.