What exactly is renal artery occlusion?

Obstruction: Renal artery is the artery that feeds the kidney. If occluded, it is shut down. The kidney will not necessarily shut down however. It may still get its blood supply from collaterals (small branches) that bypass the blocked site. The kidney function will not be normal though!

Blockage / narrowing: Of renal artery caused by atherosclerotic plaque or in few instances by fibromuscular dysplasia which occurs mostly in younger women, or from a fibrous obstruction in people with nerofibromatosis.