From The Food Encyclopedia by Jacques Rolland and Carol Sherman
Also hallah. A kosher bread, sometimes colored golden brown with saffron, challah is traditionally served on the Sabbath. The bread is braided or twisted, glazed with beaten egg and sprinkled with sesame or poppy seeds. Two loaves are served at each of the three Sabbath meals, as a remembrance of the double portion of manna that fell for the Israelites in the wilderness. For certain festivals, such as Rosh Hashanah and Yom Kippur, challah may be rounded, rather than braided, and decorated with a dove wing or ladder made from excess dough. When challah is prepared, a small piece of the dough is pinched off and put in the oven to burn as a symbolic offering to the priesthood. In Hebrew, challah means "the priest's share" or "portion." Claudia Roden in The Book of Jewish Food says: "It is derived from the Hebrew word used for ‘portion' in the Biblical commandment ‘Of the first of your dough you shall give unto the Lord a portion for a gift throughout your generation.'"
