What is the scale for on a thermometer?

The C is Celsius and the F is Fahrenheit.

On a Celsius scale water freezes at 0 degrees Celsius and boils at 100 degrees Celsius.

On a Fahrenheit scale water freezes at 32 degrees Fahrenheit and boils at 212 degrees Fahrenheit because Celsius is used world wide and Fahrenheit is used only in the U.S.