The fuel in the wing tanks gets very cold during flight and cools the wing upper, and lower, surfaces in the areas of those fuel tanks. Warmer air, on the surface, can hold more moisture than the colder air at altitude. That warmer, moist air comes in contact with the cold wing surfaces and results in either condensation or, if the wing is cold enough, frost.
A thin layer of frost on the underside of the wing is not a problem but when the frost forms on top of the wing it must be removed, or allowed to melt, prior to takeoff.
I had to have our wings spray last night in Anchorage for exactly the same reason.