those are kelvin ratings on the bulb. It is representative of the color spectrum the bulb has. If you think about the color spectrum the higher numbers are going to have a spectrum that has more blue/purple wavelengths where as lower numbers will have more green/yellow wavelengths.
live aquaria has a real basic "summing things up" type article that may explain it a bit better. Good way to wrap your head around lighting.
Aquarium Lighting: Spectrum and Intensity - Part 1
There is more to it but it gets boring real fast. If you want more in depth information let me know and i can give you some articles that are more technical.