Actually Dk is the permeability coefficient of a given material.If you don't understand coefficient, it is some constant value defined by the scientists. Do you know α represents sound absorption coefficient. The same goes for Dk. It might be the name or some symbol assigned by the person who discovered it.

t as I mentioned above is the thickness of the lenses.

The whole formula emphasizes on the rate of a chosen gas (in this case oxygen) passes through the material (lenses) of a given thickness for a given gas pressure. That is why you find the mmHg there. It is a measure of pressure.
Anyway, the whole derivation of the unit of measurement is quite complex for layman to understand. What you need to know is a higher value is much better than a lower one as it enables more oxygen to reach your cornea.

Should you interested to know how the formula is derived, pm me.