A degree of frost is a non-standard unit of measure for air temperature meaning degrees below melting point (also known as "freezing point") of water (32 degrees Fahrenheit or 0 degrees Celsius). "Degree" in this case can refer to degree Celsius or Fahrenheit.
When based on Celsius, 0 degrees of frost is the same as 0°C, and any other value is simply the negative of the Celsius temperature. When based on Fahrenheit, the conversion is a bit more complicated, as 0 degrees of frost is equal to 32°F. Conversion formulas:
Daniel Gabriel Fahrenheit (24 May 1686 – 16 September 1736) was a Dutch-German-Polish physicist, engineer, and glass blower who is best known for inventing the mercury-in-glass thermometer (1714), and for developing a temperature scale now named after him.