If you take a piece of white paper into different lighting conditions, it will be an objectively different color in each situation, but our brains are clever enough to make us feel like it's still white - it's still the same piece of paper, after all. To match our experience, cameras have to do this, too, "balancing" the colors of an image so that a white object looks white under a given light, rather than some other color. And the typical unit to measure the color of a light is the Kelvin. Which is weird, because Kelvin is a unit for measuring temperature, not color. What temperature and the color in a photograph have to do with each other comes down to history and physics.