Light has a color temperature, measured in kelvin. Candlelight sits around 1900K and looks deeply orange. Overcast daylight sits near 6500K and looks blue by comparison. Human vision quietly compensates for this, which is why a white shirt looks white indoors and outdoors. A camera does not compensate unless told to.
White balance is that instruction. Tell the camera the light is 3200K tungsten and it adds blue to cancel the orange. Tell it the light is shade and it adds warmth. Auto white balance guesses, usually well, and usually badly in scenes dominated by one strong color — a room full of green foliage or a stage lit entirely in red.
Shooting raw makes white balance a decision you can defer, because the setting is stored as metadata rather than baked into the pixels. Shooting JPEG or delivering straight out of camera makes it a decision you must get right in the moment. Either way it is worth breaking the rule deliberately: leaving a sunset warm rather than neutralising it is usually the better picture.