There are about a million nephrons in the kidneys. Each nephron consists of a capsule that holds a tuft of tiny cappillaries that are called the glomerulus. After the blood is filtered in the glomerulus, it passes into the renal tubule and into the collecting system, where it is passed out of the body as urine. In FSGS there are focal lesions, meaning that not all of the glomeruli are affected, only some of them. Glomerulo refers to the glomerulus, so that you know that the disease is in that tuft of cappilaries and not in the tubule.
Sclerosis means scarring.
So put it all together and you have a situation in which, some but not all, and part of, but not the whole glomerulus is scarred. Whatever insulted the kidneys in the first place, might never be known. I never found the cause of mine. It was just called idiopathic nephrotic syndrome, which means no known reason. The description of what the doctor want's to do sounds like plasmapheresis, where the blood is removed from the body into a machine, somewhat like a dialysis machine, and the immune complexes are removed from the plasma and new plasma is returned.