A. The word Challah actually is Hebrew for loaf of bread. We begin our Shabbat meals by making a blessing over two loaves of bread. Those loaves are often called Challah, because before we bake them we must fulfill the Mitzvah of Challah.
The Torah tells us (Numbers 15:19) that when we bake bread, we must separate a piece of dough and give it to the Kohen (priest) to eat when they are ritually pure. Today we all ritually impure (we do not have the means to ritually purify ourselves) so when baking bread, we separate a piece of dough and burn it. This Mitzvah is called Challah.
The first time we observed Shabbat in history was when the Manna fell for our ancestors wandering through the desert. Every day they would go out and collect the Manna. On Shabbat they would not collect. Instead, on Friday they collected double for Shabbat. To commemorate the double portion, we have two loaves (Challahs) at our Shabbat table.
It is customary to braid the Challah that we eat on Shabbat. We braid each one with three strands, together; the two Challah’s have six strands. This signifies the six days of the week preceding the Shabbat. When we braid the Challah we signify bringing those six days together creating unity and harmony in our lives by celebrating Shabbat.