Kelvin (K) refers to the measurement of Color Temperature or Color Appearance of light produced by a bulb. 

The lower the Kelvin number the "warmer" the appearance of light. The higher the Kelvin number the "cooler" the appearance of light. Warm light has more red and orange wavelengths. Cool light has more blue and green wavelengths.

Color Temperature or Color Appearance expressed on the Kelvin scale:

3500 K or less = Warm

3500 K to 4000 K = Neutral

4000 K to 6000 K = Cool

6500 K = Daylight

Color Reference

3000 K = Standard Auto Halogen bulb

2800 K = Incandescent Indoor bulb