After reciting Kiddush, the head of the household lifts the two challah loaves and says the blessing over bread.
We use two challah loaves for a particular reason. In the desert, after the Exodus from Egypt, the manna did not fall on Shabbat. Instead, a double portion fell on Friday (see Exodus 16:4-30). By using two loaves at the Shabbat meal, we remind ourselves of the sanctity of Shabbat and that God will provide for our material needs.
The double portion that fell for the ancient Israelites in the desert is a symbol that the one who is shomer shabbat (a Sabbath observer) will not lose out because of his observance. One of the loaves is cut and the bread is distributed to everyone. It is customary to dip (or spinle) the slices with some salt just as the Sacrifices in the Temple were salted before being consumed (see Leviticus 2:13).