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.
