Light comes in colors. When we see daylight outside, we see white light, or as it reflects off objects, we see the color of the object, but is it white? Our brains are able to white balance light to look normal quite well in most environments. Even though Auto White Balance on your camera can do a fair job of adjusting for the variation, a more accurate balance needs to be made manually.
Light color is measured as temperature. Daylight is generally 5500 Kelvin (outdoor or blue) and incandescent is 3200 Kelvin (indoor or red). The process for the color comes from heating a block of carbon to the said color, measured in Kelvin degrees. For more details and color use this chart. Outdoor light temperatures change during the day and in and out of shade, and artificial lights, including reflectors, can run the gamut. You need to balance every time and maybe every hour.
To white balance your camera, you fill the camera frame with a white object in the light and exposure you are shooting in (it can change during the shoot) then manually set the White Balance, (from your camera directions) and you’re good. If you have mixed colors of light such as daylight shining in on your incandescent lit studio shot, you will have to eliminate one or the other or balance the light with gels .
“White objects” come in many hues. To get the most accurate you need a professional balanced card. They may seem expensive compared to a white sheet of paper, but it will make you or a client much happier because the white balance will be spot on.
There are exceptions to the rule as with much of filmmaking. You can trick the camera for a false color or enhance a color for your film. Use a colored card opposite of the color you want and the camera will “balance” in the direction you want. With practice, you could blow them away with your cinematography.