This is the central candle on the altar. There are three candles used in our tradition; one for the Maiden, one for the Lady, and one for the Lord. The Maiden candle is in the central place, because the Maiden comes before the Lady (a woman is a girl before she is a wife or mother.) Also, the Maiden stands alone. She has no need of any other to be complete. This candle reminds us of that. It's also the candle that is used to light the corner candles, the incense, or anything else that needs to be lit during the celebration.
The maiden candle is almost always white, to show the maiden's purity. It may be other colors, though, depending on the purpose of the celebration. Still, white is a very good all-purpose color, because white light contains all the other colors within it.