This stands for the measurement Kelvin.  It is expressed in degrees Kelvin.  This is used to rate the color temperature.  Lights in the range of less than 4300K are considered warm (red/orange/yellowish), 5000-6000K is considered daylight bulbs (yellow-white to blue-white) and 10,000K is typically very blue-white.  Those of 12,000 to 20,000K (rare but available) are very blue and are best used for very very deep water tanks that is suplemented by a 6500-10,000K bulb and/or VHO fluorescents for color correction to a more "natural" tone.