You shall place show bread on the table, before Me, at all times. (Exodus 25:30)
One of the vessels of the tabernacle was the Shulchan (table), on which the “show bread” was placed. (Older translations may have “shewbread.”)
In Hebrew, the show bread was called “lechem hapanim” (“bread of faces”) because of its open box-like shape with numerous surfaces. The details of this mitzvah are found in parshas Emor (Leviticus chapter 24): there were 12 loaves, which were made from fine flour. These were placed on the Shulchan in two stacks of six. They were arranged on Shabbos and the previous week’s loaves were removed and eaten by the kohanim. (Other details are found there and discussed in greater depth in the Talmudic tractate of Menachos.)
The lechem hapanim was made by a family of kohanim named Beis Garmu, who zealously guarded its secrets so that it could not be replicated for idolatrous purposes. Beis Garmu also never kept flour suitable for lechem hapanim in their homes so that no one should suspect them of making it for their own use. (See Yoma 38a. The Talmud in Shekalim 14a explains the secret of Beis Garmu. Apparently, the lechem hapanim was baked in special molds in order to give the loaves their unique shape. The members of Beis Garmu were able to remove the lechem hapanim from these molds without the bread sticking to the molds and breaking.)
The basis of this mitzvah is that bread is the physical staple of man’s existence (although, as Deuteronomy 8:3 reminds us, man does not live on bread alone, he also requires God’s word). God incorporated bread as part of the Temple service on His behalf, in exchange for which He places His blessing on man’s sustenance. The Sefer HaChinuch explains that whatever man uses to serve God will be blessed. He cites the Talmud in Rosh Hashana 16a, where it explains that the omer is brought on Passover as a sign of blessing, the water libations are poured on Succos as a sign of blessing for the rains, etc.
The obligation to set the lechem hapanim on the Shulchan only applies to male kohanim (priests) at a time when there is Temple service. It is discussed by the Talmud in Menachos 94a-100b. It is codified in the Mishneh Torah in the fifth chapter of Hilchos Tamidin and it is #27 of the 248 positive mitzvos in the Rambam’s Sefer HaMitzvos