Every few months or so, your parents would get a letter from school and you'd have to explain how you got a "C-" in gym. A grading certificate is kind of like that, only for bushels of wheat instead of for kids.
A grading certificate represents an official document attesting to the quality of a commodity. An inspector has taken a look at the meat or the corn or whatever, and has certified that it meets a certain standard.
The certificate will include basic information about the commodity, such as where it's from, and that it contains the correct amount of the item. It also includes a grade, giving an assessment of its quality. Better quality means higher prices.
These certificates are important for trading purposes and come into play on the commodities market.