Frost buildup on the inside surfaces in the freezer compartment.
When (moist)air from outside comes into the freezes, frost is build up on the inside surfaces of the freezer compartment (see to images below).
This is mainly caused by a not closed freezer door. What happens in these cases is that freezer food, food boxes or other frozen material is placed and stacked in such a manner that the door cannot fully close. So the door may seem closed, but when opening it no vacuum/resistance is experienced. In other words the door opens too easy.
It is also not advised to clean the inside of the freezer with a damp or wet cloth. This will also create similar frost symptoms
If the previous points are not the case please check the door rubber for damages. (some refrigerator models have open holes on the corners of the rubbers, this is normal and used to deflate the door rubber when closing the door)