It means when a child urinates, a small amount of urine stays in his/her bladder. When it gets to a point, it will cause infections. It's common among children between age 0 and 4 years old. But 75% of children will outgrow it after 4. My baby needed to take medication for 1 1/2 year to prevent infections. But she has grown out of it and is completely fine now. Thank goodness!
