What is the meaning of the K in the temperature?

K is the abbreviation for Kelvin, a unit of temperature. It's more or less the unit used for scientific purposes and not commercial use (usually the US weather reports are in Fahrenheit degrees and not Kelvins).

To convert from Celsius to Kelvins: [K] = [°C] + 273.15

To convert from Fahrenheit to Kelvins: [K] = ([°F] + 459.67) × 5⁄9