Challah is a special bread in Jewish cuisine, usually braided and typically eaten on ceremonial occasions such as Shabbat and major Jewish holidays (other than Passover). Ritually-acceptable challah is made of dough from which a small portion has been set aside as an offering.
Most traditional Ashkenazi challah recipes use numerous eggs, fine white flour, water, sugar, yeast, and salt, but "water challah" made without eggs and having a texture not unlike French baguettes also exists. Modern recipes may replace white flour with whole wheat, oat, or spelt flour or sugar with honey or molasses.
Egg challah sometimes also contains raisins and/or saffron. After the first rising, the dough is rolled into rope-shaped pieces which are braided, though local (hands in Lithuania, fish or hands in Tunisia) and seasonal (round, sometimes with a bird's head in the center) varieties also exist. Poppy or sesame (Ashkenazi) and anise or sesame (Sephardic) seeds may be added to the dough or sprinkled on top. Both egg and water challah are usually brushed with an egg wash before baking to add a golden sheen.
Something, often what is called a Challah board, is under the Challah, although a simple napkin is acceptable if nothing else is available. The "under" and "covering" are to remind of the dew that fell, before and after the manna. It also serves as a cutting board for slicing the Challah.
The Torah requires that Temple sacrifices to God be offered with salt. Following the destruction of the Second Temple, Rabbinic literature suggested that a table set for a meal symbolically replaces the Temple altar; therefore, the blessing over food should only be recited with salt present on the table. Should one eat a meal without performing a commandment, the covenant of salt protects him.
The earliest written source for this custom is the sefer Ohev Yisrael by Rabbi Avraham Yehoshua Heshel, the Apter Rav, written in the 1800s. He calls schlissel challah "an ancient custom," and offers several kabbalistic interpretations. He writes that after spending forty years in the desert, the Israelites continued to eat the manna until they brought the Omer offering on the second day of Passover. From that day on, they no longer ate manna, but food that had grown in the Land of Israel. Since they now had to start worrying about their sustenance rather than having it handed to them each morning, the key on the challah is a form of prayer to God to open up the gates of livelihood.