Kidney cysts are round pouches of fluid that form on or in the kidneys

Kidney cysts are round pouches of fluid that form on or in the kidneys. Kidney cysts can be associated with serious disorders that may impair kidney function. But more commonly, kidney cysts are a type called simple kidney cysts — noncancerous cysts that rarely cause complications.
It's not clear what causes simple kidney cysts. Typically, only one cyst occurs on the surface of a kidney, but multiple cysts can affect one or both kidneys. However, simple kidney cysts aren't the same as the cysts that form with polycystic kidney disease.
Causes
It's not clear what causes simple kidney cysts. One theory suggests that kidney cysts develop when the surface layer of the kidney weakens and forms a pouch (diverticulum). The pouch then fills with fluid, detaches and develops into a cyst.

Symptoms
Simple kidney cysts typically don't cause signs or symptoms. If a simple kidney cyst grows large enough, symptoms may include:
Dull pain in your back or side
Fever
Upper abdominal pain

Kidney cysts can occasionally lead to complications, including:
An infected cyst. A kidney cyst may become infected, causing fever and pain.
A burst cyst. A kidney cyst that bursts causes severe pain in your back or side.
Urine obstruction. A kidney cyst that obstructs the normal flow of urine may lead to swelling of the kidney (hydronephrosis).