Purity is the condition or quality of being pure; freedom from anything that contaminates, pollutes, etc., freedom from any admixture or modifying addition; cleanness; freedom from guilt or evil; innocence. Purity is completely, one hundred percent essence without any contamination.
God is the only pure, uncontaminated being. He can never be defiled in any way because He cannot change. He is pure in His being and cannot in any way be affected by any kind of source that would change His purity. He is complete and perfect and is pure God, no additives or fillers. Maybe purity is better understood as the absence of anything foreign to what He is.