The term "Frost Free" is used to indicate that the refrigerator has an electric heater that will come on and melt the ice formed on the coil in the freezer section. Frost is formed from moisture rich air coming in contact with a very cold surface, such as a metal panel or product stored in the freezer.
In most cases the condition of the door gaskets need to be checked. I would suggest the dollar bill test. This is a test that any service technician would use to check the condition of the door gaskets ability to seal properly. Take a dollar bill, place it between the door gasket and the door, close the door and pull on the dollar bill. There should be tension on the bill while pulling it out. If the bill pulls out easily, the gasket does not have the ability to provide a good seal in that area. Please keep in mind there is a lot more to frosting than door gaskets.