He made him ride on the high places of the earth, that he might eat the increase of the fields; and he made him to suck honey out of the rock, and oil out of the flinty rock; Butter of kine, and milk of sheep, with fat of lambs, and rams of the breed of Bashan, and goats, with the fat of kidneys of wheat; and thou didst drink the pure blood of the grape. Deuteronomy 32:13-14
The Hebrews regarded the fat as the choicest and best part of the animal, not to be eaten by men but to be burned upon the altar as an offering by fire to the LORD. "The two kidneys, with the fat that is on them at the loins" are especially mentioned in the law of sacrifice recorded in Leviticus 3 and 7. Hence the best part of anything was called its "fat."
"The fat of the land" which Joseph promised to his father and brothers meant the choicest products of the land (Genesis 45:18). "All the best of the oil, and all the best of the wine and of the grain" (Numbers 18:12), "the finest of the wheat" (Psalms 81:16; 147:14) are phrases in which "best" and "finest" represent the Hebrew word for "fat." Therefore the strange expression, "the fat of kidneys of wheat," does not imply that there are any kidney-shaped grains of wheat; it means the kidney-fat, the best of the fat, the best of the best, of the wheat (Deuteronomy 32:14). [Donna: We would say "cream of the crop."]
(The Bible Word Book, Bridges & Weigle pg 195)