Both IU/d and nmol/L are measurements of molecules. nmol/L is "nano-moles per liter", and is a measure of concentration: https://en.wikipedia.org/wiki/Molar_concentration

"IU" is a measurement common in pharmacology, and has to do with trying to standardize how much mass/volume of a substance will give a particular biological effect. The "/d" is per day.

In summary, the article is saying that, based on a meta analysis[1] of previous research, we have misunderstood or misreported some of these measurements. They suggest both the safe dosage and the effective dosage for Vitamin D should be much higher, and argue that this is a cheap and effective way of treating problems associated with Vitamin D deficiency.

[1] https://en.wikipedia.org/wiki/Meta-analysis

(edited for spelling)