Eighteen-twenty minutes before sunset on Friday the Shabbat candles are lit in Jewish homes. In houses of married couples and families, the wife and mother lights the candles on behalf of the family. In the case of a single man or woman, s/he is obligated to light Shabbat candles in their home.
In most cases in Judaism when a blessing is recited over performing a commandment, the blessing is recited first and the action is performed afterwards. Examples are Megillat Esther- the scroll read on the minor holiday of Purim, whereby the blessing over reading the scroll is recited and immediately after, the scroll is read from or the commandment of taking the four species on the festival of Sukkot- firstly the blessing is recited and then the bundling of the species and their shaking in six directions is performed.
In the case of the Shabbat candles however, the action of lighting the Shabbat candles is performed first and the blessing is recited after the act. The reason for this is that when the woman lights the Shabbat candles, she is accepting the Shabbat day upon herself, with all the restrictions it includes, one of which is lighting a flame- if she were to bless first, she would not be able to then light the candles!
Therefore, after lighting the candles the woman covers her eyes and recites a blessing that translates to mean,
“Blessed are You, our G-d, King of the Universe, Who has commanded us to kindle the light of the Shabbat.”
After reciting the blessing it is customary for the woman to recite silent blessings for the welfare of her family. In Jewish tradition, this time is considered to be an auspicious time and as a result women take advantage of this and use the time to request from G-d all the things needed for each and every family member.
The lighting of the Shabbat candles signifies the descent of the Shabbat day upon the home. Of course, it goes without saying that whether or not family members light the Shabbat candles Shabbat will arrive when nightfall occurs because Shabbat is not dependent on mankind. The actions of the women of Israel ushering in the Shabbat day through lighting the candles demonstrates her eagerness and that of her family to draw close to G-d.
There is a custom for the man of the house to set up the candles for his wife so that he can have some kind of participation in this beautiful commandment. Some men even light and blow out the candles before their wives light so as to make it easier for their wives to light the candles. Through these actions, the man is enhancing the harmony between him and his wife, bringing a peaceful atmosphere into the house.