The colour temperature of an image is essentially how warm or cool it feels (for the technically minded, it is measured on a Kelvin scale, but lets not worry about that here).
An image taken under a clear blue sky will have a 'cool blue' feel to it. However, one taken at sunset or under tungsten light will have a warmer 'orange' feel to it.
The nice thing about colour temperature is that we can change it to suit our subject. If your image has been shot in RAW, then it is a simple case of moving a slider in Lightroom to make the photo warmer or cooler (it can be done on a JPEG, but the results will be much less consistent).
The slider is in the Develop Module, at the top of the Basic section. Move the 'Temp' slider to the left to get more blue tones in to your image, and to the right to get more yellow.
The images of the musician on this page originate from the same RAW file that has been treated to have warmth in one version, and a cool feel in another.
Once you have adjusted the colour temperature, you can then take the image even further from its original RAW version. You can see this effect in the following two images of a lighthouse. Quite a lot of further editing has taken place on these photos once the colour temperature had been adjusted.