Help me out here: what is renal scarring?

FSGS, probably. Focal segmental glomerulosclerosis is the disease usually described this way. Diagnosed by biopsy. However, other kidney diseases when deteriorate end up finally with scarring too, called global glomerulosclerosis. In short, renal means kidneys' related, scarring desscribes filtering units as being scarred up and shriveled partially or totally hurting the ability to filter & causing protein leak.