Historical Facts
There are two words for bread in Hebrew: lechem and challah. Lechem is the everyday bread and challah is the special, usually white egg bread reserved for the Sabbath. Challah is also the word that refers to the portion of dough set apart for the high priests in the Temple of Jerusalem.
One of the three commandments incumbent upon women, "taking challah," evolved sometime following the destruction of the Temple by the Romans in 70 C.E. Following the rising of the dough, women would separate a piece and burn it to remind them of the offerings to the Temple.
For nearly two millennia challah has symbolically replaced the sacrificial offerings. All challah that is baked today is kosher only if "challah has been taken." You'll often see that sign in a kosher bakery. Jews seem to be masters at imbuing ordinary acts with symbolism - or creating symbols out of almost anything.
Challah in a traditional Sabath setting

Lighting candles on the Sabbath