The Dk value measures oxygen permeability, the Dk/t value measures oxygen transmissibility of a contact lens. Dk/t is calculated from the oxygen permeability (Dk) of the material and the thickness (t) of the contact lens. Usually the Dk/t value refers to a certain center thickness of a lens with a certain power, therefor making it easier to compare lenses.
In general, the higher these values are, the better is the oxygen permeability/transmissibility of the material. A high level of oxygen flow to the cornea is important as it improves wearing comfort and longer term tolerance.