For a technical explanation of white balance you can read this wiki article http://en.wikipedia.org/wiki/White_balance
A less technical explanation of white balance is, settings that will try to correct colours depending upon the ambient lighting. For example a sheet of white paper will look different when lit by a filament light bulb, fluorescent lighting or natural sunlight. So most cameras have white balance settings for these different situations (or an auto setting so you don't have to worry about it).
If you shoot in RAW and not JPEG you have more options to modify the colour temperature (white balance) in post processing.
Nikkor AF-S DX; 18-105 f/3.5-5.6G VR, 55-300 f/4.5-5.6G VR, 35mm f/1.8G
Speedlight SB-700 http://keystrokesukimages.com