That is refering to the colour spectrum of a given light bulb, for example 10,000k, 14,000k etc etc..
The "K" is refering to the "Kelvin" of the bulb. Some examples are...
10k - A more yellow type of light emitted
14k - A more whitish light emitted
20K - A more bluer light is emitted..
The most popular bulb to use i think is the 14k as it gives a nice white light and also a little ammount of blue aswell..
Hope this helps
Niko