Different light sources have different color temperatures. Indoor lamps can look warm and orange, while open shade can appear cool and blue. White balance tells the camera how to interpret those colors so that neutral objects look closer to neutral.
Choose a useful starting point
Automatic white balance works well in many changing scenes. Daylight, shade, cloudy, and indoor presets can give more consistent results across a set of photographs. A custom white balance uses a neutral reference under the light in the scene.
Color temperature is measured in kelvins. A lower setting makes the rendered image cooler; a higher setting makes it warmer. The exact display can vary between cameras, so judge the result on the image rather than relying on a number alone.
Keep color consistent
If you photograph a sequence under the same light, a fixed preset or custom value can prevent color from shifting between frames. RAW files preserve more flexibility for later adjustment, while JPEG processing bakes in more of the camera's chosen look.
Check skin tones and known neutral surfaces. A technically neutral image is not always the most pleasing one; warm sunset light may be part of the atmosphere you want to preserve.