What is the "Yoseres"?
Rashi: The lobe of the liver (or the diaphragm). 1
It is a wall of fat that divides between the limbs that govern one's breathing and those that deal with the food (Sifsei Chachamim).
Where exactly is the Chelev that is described in this Pasuk?
Rashi: When the animal is alive, the Chelev that is on the kidneys is on top of the flanks, which in turn, are underneath the loins. It is synonymous with the white that can be seen on top of the flanks, whereas the Chelev on the lower part of the flanks is covered by the flesh.
What does the last part of the Pasuk mean? What does one remove and what does one not remove?
Rashi: One removes the Yoseres together with a bit of the liver, but leaves intact the bulk of the liver and the kidneys.