When applying herbicides what does ai stand for eg kg ai per ha?

It stands for "active ingredient." For example, "Roundup Original Max" brand of glyphosate herbicide contains 660 grams per liter (5.5 pounds per US gallon) of the active ingredient glyphosate. So if the recommendation is for 1.32 kilograms a.i. per hectare, you would prepare the equipment to apply two liters of product per hectare.

Remember to always read, understand, and follow all label instructions BEFORE using any pesticide!