It turns out that White Balance is a difficult term to define for most people. Here's my try.
White balance is the process of adjusting color casts, so that objects which appear white to human eyes/brain are rendered white in the photograph by the camera.
So what the heck does that mean? Photography is all about light. As we know, if we examine light, which from the sun appears white to our eyes, it's made up of a whole spectrum of colors, each with its own color temperature which describes their color. Cameras need the ability to render the colors of the scene as the eye sees them and that isn't necessarily easy.
In the past with film photography, while most photographers were aware of this, they took color rendition for granted, how the color temperature of the light falling on their subject and how their film might render it. We bought film for particular uses, daylight, vs. tungsten, for example (outdoor vs. indoor).
Now in the world of digital photography, how the colors are rendered has become very important as we don't have film choices which take care of it for us. Moreover, as black and white photography is rarely used by the majority of digital photographers, other than pros and advanced amateurs generally, color rendition has become more important than ever. (Actually color rendition is important in Black and White photography too, as various shades of gray are renderings of the colors of the scene.)
It turns out that color rendition of a scene and white balance go hand in hand. If your white balance is correct, the color in your photograph will be true to life.
Each light source has its own color temperature. Even light coming from the sun at different times of the day has different color temperatures. Simplifying, color temperature is basically the hue and intensity of a particular light source, measured in degrees Kelvin.
Think about how a scene looks at noon vs at sunset. The color temperature of the sun in bright daylight is different than when its filtered by light clouds, and different again when filtered by heavy clouds, for example.
In each case while your eyes/brain know what's white and will make white look white for you when possible, the camera can't do that without some kind of electronic tweaking of the color rendition produced by the sensor and that's the camera's white balance control circuit.
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