The Kohein Gadol (High Priest) was to offer a korban mincha twice each day, once in the morning and once in the evening. A tenth of an ephah of fine flour was divided between these two offerings. Each half was used to make six loaves, for a total of 12. These loaves were boiled, baked, fried in a pan with oil, then broken into pieces. The Talmud in Talmud Menachos, page 50b, discusses the order of the baking and the frying. Rashi there (s.v. t’afina rabbah) says they were baked, fried, then baked again. (An ephah was 22 liters or about 5.8 gallons; one-tenth of that is a little over two liters or two quarts.)
The basis of this mitzvah is that the Kohein Gadol was the representative of the people to God. He worked on their behalf to affect atonement. But it wouldn’t do to have him just show up on Yom Kippur, asking God for forgiveness. Rather, the Kohein Gadol had to maintain his own ongoing relationship with God. And so, just as the nation gave two daily communal offerings, the High Priest was obligated in two daily personal offerings. The personal offering forged a closer bond between him and God than his own share in the communal sacrifices.
There is a difference of opinion as to whether bringing the two daily communal sacrifices was one mitzvah or two (see Mitzvah #401, in parshas Pinchas). Here, all authorities agree that the Kohein Gadol’s twice-daily flour offerings are a single mitzvah since the Torah specifies to offer half in the morning and half in the evening.
This mitzvah only applied to the Kohein Gadol in Temple times. It is discussed in the Talmud in tractate Menachos, on page 50b-51b. It is codified in the Mishneh Torah in the third chapter of Hilchos Tamidin. It is #40 of the 248 positive mitzvos in the Rambam’s Sefer HaMitzvos. (The verse in which this mitzvah appears is Leviticus 6:13. There’s no significance to that, we just felt like mentioning it.)