A test sample of the flour is burnt in a lab, and the residual ash (or residual mineral content) is measured. The more of the whole wheat grain was in the flour, the more ash there will be left behind. So the higher the rated ash content of a flour, the more “impure” the flour is seen to be, because it contains more of the “brown stuff” from the wheat. (Though nowadays, higher ash contents have become more desirable, indicating as it does more of the whole grain being present in the flour.)
The ash content is expressed as a percentage of how much ash there was per 100 grams. When applied to flours, it’s converted to a whole number, with the number representing the weight in grams of potential ash, were the flour to be burned. It does not mean there is actually ash in the flour, as obviously no one would buy it.