The Texas Cottage Food Law website has the best explanation:
Non-potentially hazardous foods are foods with a low water activity and low pH level that inhibit the growth of dangerous micro organisms. Basically it means non-perishable foods; foods that you would not normally keep in the refrigerator. Foods that are perishable, such as cheesecakes, or custard fillings, or meringue pies, are not covered in the law. To sell foods that require refrigeration, you must contact your local Health Department and find out how to comply with their requirements. You would not be able to sell those foods from home.