HISTORY and BACKGROUND (optional reading)
The phrase "White Balance" comes from color television. It is how we set a color TV camera to give accurate color in any kind of light. In Hollywood we also speak of gray and black balance and have controls for these, but not in consumer video or still cameras.
We set white balance while pointing a video camera at a white card.
In the 1950s we adjusted the gains of the three Red, Green and Blue tubes with screwdrivers until we got equal levels (balance) on a component waveform monitor or a dot in the middle of a vectorscope. Today we just press the WB button while pointed at a white card.
We use white cards, pieces of paper, backs of business cards or even white T-shirts. It's no big deal, so long as it's white or neutral. Whites are easy to find in the field; we never trust grays.