Frost inside of a car's windows means that low temperatures caused the water vapor in the air to condense and freeze on the glass. This phenomenon happens especially in the winter, after a sunny day and freezing night.
During the day, the sun warms the car's interior, causing the snow or rain carried into the vehicle to evaporate. Later, when the temperature dips below freezing, the water condenses on the cold windows and freezes. So, to avoid frost on the inside of the windows, take measures to decrease the humidity in the car, such as leaving the window slightly open to allow it to escape.