The kidneys filter the blood and move waste into the urine. Urine forms in the kidneys and flows into the ureters. These are tubes that drain urine into the bladder.
When the kidneys and collection system form as a fetus is growing, there can be a duplicate or extra-set of connections. These usually do not show any symptoms. They would show up on a scan (such as an ultrasound) as an "extra" or duplicate collecting system.