The higher the mineral content, in other words, the higher the number, the more outer parts of the wheat grain are present in the flour. As the outer layers are much richer in minerals than the inner endosperm. The higher the number, the darker it is and the higher is the fiber and mineral content. To make it short, the higher the number, the healthier.
Here is a table from Wikipedia on wheat flour type numbers
|Ash||Protein||Wheat flour type|
|~0.8%||~14%||high gluten flour||812||80|
|~1%||~15%||first clear flour||1050||110|
|>1.5%||~13%||white whole wheat||1600||150|
This table is only a rough guideline for converting bread recipes. Since the American flour types are not standardized, the numbers may differ between manufacturers.