Because it is forbidden to kindle lights on Shabbat, candles are lit prior to Shabbat so that there should be plenty of light to fill the home. One of the reasons that we light a minimum of two candles is to represent the two different ways that the Commandment to keep Shabbat was transmitted in the Torah. The first time we are instructed to "remember" the Shabbat and the second time to "guard" it. Each candle is symbolic of these two ideas.
There is a beautiful custom that many people have which is to light an additional candle for every child in the family, because the flame of a candle is compared to a Soul, just as a flame is never still so too does the Soul continuously strive to "reach up" to Hashem. However, one of the repercussions of the sin of Adam and Eve is that death was introduced in to the world. By lighting Shabbat candles we are, symbolically, reintroducing life in to the world because Shabbat brings with it an extra Soul, called the Neshama Yeteira, for those that keep it.
