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(temperature scale starting at absolute zero)

A kelvin (K) is a (SI) unit of temperature, generally used to indicate an amount of temperature above absolute zero, i.e., above the physical lower bound on temperatures. The unit size is borrowed from the degree Celsius (centigrade) temperature scale (which pins zero degrees Celsius to water's freezing temperature and 100 degrees Celsius to water's boiling point for a unit of 9/5 a degree Fahrenheit) but among SI efforts is a more careful definition, based upon measurable physical values such as the speed of light and the Planck constant. Unless explicitly referring to a temperature difference, a number of kelvins refers to that number of kelvins above absolute zero. Physical relations tend to be based upon temperature scaled from absolute zero and kelvins are a convenient scale in the sciences.

kelvins Celsius Fahrenheit
0 -273.15° -459.67° absolute zero
255.372 -17.8°
273.15 32°
294.261 21.1° 70°
5778 5505° 9941° surface temperature of the Sun

Regarding writing style, the unit kelvin is not capitalized, but the abbreviation K is capitalized: not the usual practice for SI units, but presumably a means to avoid potential confusion with "k for kilo". The term Kelvin scale for referring to the temperature scale in kelvins is capitalized. Terms like "MK" for millions of kelvins (or "kK" for thousands) are used, but it seems to me they are often avoided and readers aren't used to them: mK (millikelvin) and μK (microkelvin) seem more common, seen when referring to temperatures below 1 K. Unlike Fahrenheit and Celsius/centigrade (and Rankine), kelvins are not referred to as degrees, i.e., you do not write 100°K but 100 K. Referring to kelvins "a type of degree" (e.g., saying "10 degrees Kelvin") was formerly the practice, but SI standardization efforts declared in 1967 that the unit is a kelvin rather than a degree kelvin, so you say "10 kelvins".

Further reading:

Referenced by pages:
Arcminute Microkelvin Imager (AMI)
black-body radiation
Boltzmann constant (k)
cold spot
far infrared (FIR)
Fermi sea
H-R diagram (HRD)
Origins Space Telescope (OST)
Planck units
stellar atmosphere
ZTF J1539+5027