Before defining manual white balance, it makes sense to address the question of what white balance is. White balance in photography is a feature of digital cameras that allows you to adjust the color balance of an image to achieve neutral whites.
In a sense, you’re removing unrealistic colors so that objects in the image that appear white in person also appear white in the picture. Camera white balance usually isn’t an issue as digital cameras now come with built-in auto white balance settings that do a pretty good job of identifying colors in an image and adjusting them accordingly (under the dark glow of an overcast day or the orange light inside of a living room, for example).
However, sometimes you need to override a camera’s white balance to set the white balance manually. To achieve the white balance you’re looking for, you can use a white balance card or take a photo of something white in the same light that you’ll be photographing your subject.