Normally once urine is produced in your kidneys, it flows down to the bladder using 2 tubes called the ureters. Urine is prevented from returning to the ureters by a valve mechanism at the junction of the ureter and bladder. Sometimes this valve fails and urine flows back and forth. This is called reflux. In some cases, this causes urine stagnation and infection and leads to reflux uropathy.
Attribution: Avallain