White balance is a process that your camera carries out in order to ensure the whites that are seen with they eye appear as white as possible in the final digital image. On my Nikon D3200 I have multiple white balance settings such as: Auto, cloudy, cool-white fluorescent, direct sunlight, flash, incandescent and shade. The auto function allows the camera to decide white preset to use, however this can sometimes leave blue or green casts on the photo when taken. For this reason the manual settings are often used to decide on the right white balance for a particular object or scenario.
Here of some examples of what happens if you change the white balance settings:
As you can see some of the above pictures have a blue cast on this which was avoidable by using other, more appropriate settings. The auto mode worked quite well here, selecting direct sunlight as its white balance which I agree with.
It is also possible to manually set your white balance using a grey card, often used by professional portrait photographers.