Scarring occurs during the healing process after damage to living tissue and cells. Scars are an imperfect process for damaged tissues to heal. Scarring usually does not cause pain. It can on rare occasions lead to impaired function of an organ if the scarring is extensive.
When this occurs in the kidney, it is usually the result of an infection within the kidney tissue. Very often, this occurs from a urinary tract infection. There would rarely be any related pain or other symptoms.
Scarring may be seen on imaging of the kidney (ultrasound, CAT scan, or MRI) without the person ever knowing they have scarring.
Another much less common cause of kidney scarring is a blood clot that prevents oxygen getting to a part of the kidney. This leads to damage of the tissue and the formation of scars.