It came to pass on the sixth day that they gathered a double portion of bread … (Exodus 16:22)
A double portion of manna would fall on Friday as it is prohibited to carry on Shabbat. Since the Torah refers to the manna as bread (Exodus 16:4) the Talmud states that we commemorate this by using two loaves of challah (plural challot) at our Shabbat meals (Shabbat 117b).
The Shulchan Aruch (Code of Jewish Law) states that the loaves used on Shabbat and Yom Tov must be whole and unsliced (Orach Chaim [OC] 274:1). Rabbi Chaim Mordechai Margoliyot (d. 1818) cites a minority opinion states that if a piece is missing which is less than one forty-eighth of the total loaf, it may still be used (Shaarei Teshuvah ibid. 1). Rabbi Yehoshua Neuwirth rules that if two challot baked together in the oven become attached they may be broken apart and still count as two whole loaves (Shmirat Shabbat KeHilchato II 55:6).
The person making the blessing should hold both loaves with all ten fingers touching them (OC 167:4). Rabbi Moshe Isserlis (d. 1572) ruled that on Friday night the bottom challah is broken but at all other Shabbat and Yom Tov meals, the top challah is used (Rema, OC 274:1).
Rabbi Avraham Gombiner (d. 1682) stated that the head of the household should mark (but not cut) the challah at the place where they intend to cut (Magen Avraham 274:1) before making the blessing (Chief Rabbi’s siddur page 754). After cutting they should first eat before giving to their wife and then guests, unless each person has their own two challot (OC 274:3).
Each piece should be dipped in salt as a reminder of the salt used on the altar in the Temple (Rema, OC 167:5). Challah should not be passed from hand to hand as this is a sign of mourning (OC 167:18) and it is improper to throw it (OC 171:1)
Just as the manna provided by G-d sustained the Jewish people in the desert, so too the bread we eat reminds us of the bounty we enjoy today and the gratitude we should show for everything we have.