White balance is one of those seemingly hard-to-understand, however, for most practical purposes extremely simple photographic concepts. The only reason why it’s difficult to grasp at first is because we seldom perceive it. And phenomena that we are not normally aware of can be a bit confusing. So, let me simplify it for you.
Our brains have evolved in such a way that we would perceive colours most of the time as constant. Life is such so much simpler if a white piece of paper always seems to be white and your friend’s skin colour appears the same irrespective of you sitting at a table in the sun or at night illuminated only by candles.
But the truth is that the white paper is not ‘objectively’ white and your friend’s skin colour changes! If you think about it, the reason is obvious. The colour of objects depends on the colour of the light that illuminates them. (After all, it’s hardly surprising that white paper looks blue if it’s illuminated by blue light.) And believe it or not: various light sources that appear white to our eyes (and brains) are actually coloured.
Sunlight, e.g. contains relatively more blue light than red.
Candlelight, on the other hand, contains more red light than blue.