Frost is simply frozen dew. Usually on lower temperature refrigerant applications, the suction line and part of the compressor's head will get cold. The part of the compressor's head that gets coldest is where suction vapors enter before they are compressed.
These parts often get cold enough to reach the dewpoint of the surrounding air. When the air comes into contact with the cold suction line and compressor head, water vapor in the air is cooled to its dewpoint and condenses on the suction line and compressor head. (See Figure 1.)