Vesico refers to bladder and ureteral refers to the ureter - so vesicoureteral reflux means that urine is refluxing or getting backed up.
Normally, urine flows in one direction - it starts in the kidneys, goes down into the bladder and when the bladder is full, urine flows out of the body through the urethra.
In vesicoureteral reflux, there is some obstruction in that path which causes pressure to build up and a current of urine actually pushes backward from the bladder into the ureters and kidneys.
There are two types of vesicoureteral reflux, or VUR. Primary vesicoureteral reflux is the most common type and happens when a child is born with a defect at the ureterovesical junction, which is the spot where the ureter enters the bladder, which also acts as a valve preventing urine from pushing back from the bladder into the ureter.
Normally about two centimeters of the ureter sticks into the bladder wall, allowing urine to flow into the bladder, but as the bladder fills up and stretches, it also stretches that section of the ureter and presses it against the top of the bladder, causing the ureter to close shut.
If the tube isn’t long enough though, that small piece of the ureter doesn’t stretch very much, and it stays open even when the bladder fills with urine. In that situation, as the bladder pressure builds with more urine, it starts to go back up the ureters.