In the Torah, God commands Jews to observe and remember the Shabbat through words, actions and thoughts.  It is believed that the lighting of two candles by the woman of a household symbolizes this commitment.  Because Jews are forbidden from kindling a fire during the Shabbat, the candles were historically lit eighteen minutes before sunset to allow enough time for the process.  Modern conveniences make lighting candles much simpler today, so the eighteen minutes serve as a meditative transition between the weekday and Shabbat.