White balance is the colour temperature of a light source.You can use it by setting your camera to the symbol M (manual exposure) depending on your camera this can be done by twisting the button on the top of the camera also known as the mode button or a dial.
It is used to get a better quality picture and does so by considering the lighting needed for the picture however you can set this e.g its cloudy so you would set the white balance to cloudy (6000k) to get a better quality picture,it does this by adding more light or less light to the picture. Also you could keep the white balance on automatic and it does it for you but it is better to do it yourself.