Sometimes kidneys are no longer able to filter and clean blood. This can cause unsafe levels of waste products to build up. This is known as kidney (or renal) failure. Unless it is treated, this can cause death.
How the kidneys filter waste from your blood National Institute of Diabetes and Digestive and Kidney Diseases, National Institutes of Health
The kidneys are 2 bean-shaped organs, each about the size of a fist. They are found in your back on either side of the spine. Healthy kidneys clean waste products from the blood by making urine. They also balance the amount of certain elements in your blood (such as sodium, potassium, and calcium), and make hormones that control blood pressure and red blood cells.
Kidney (renal) failure is when kidneys don't work as well as they should. The term "kidney failure" covers a lot of problems. These problems can result in kidney failure:
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Urology Care Foundation - What is Kidney (Renal) Failure?