White balance is the color temperature of your photo.  You can either shoot in auto white balance or adjust the kelvin setting.  

I prefer to shoot in auto because I shoot in RAW, so I’m able to adjust the color temperature in white balance.  Some photographers choose to have more control in the camera.  You can make a photo warmer (more yellow) or cooler (more blue). 

If you’re shooting at dusk, skin tones can be a little more blue (even in auto mode) and adjusting the kelvins can make skin tones look better.  That’s probably the only time I don’t shoot in auto!

Check out this little video I made to illustrate the difference between 2500K and 10000K!