Concise Bible Dictionary:
Bovine Kidneys
These, “with the fat thereof,” were often burned on the altar in the sacrifices (Lev. 3:4-15; Lev. 4:9; Lev. 7:4, and more. The same word, Kelayoth is translated “reins,” (which signifies “kidneys”) when used symbolically of the inward feelings and affections.