Okay kiddo, imagine that your bladder is like a balloon that holds pee. Normally, when you feel like you need to go pee, your brain tells your bladder to squeeze the pee out so you can go to the bathroom. But sometimes, the bladder doesn't work as well as it should and doesn't squeeze enough to get all the pee out. This is called an underactive bladder.
It's kind of like trying to blow up a balloon but you can only blow a little bit of air at a time. You might have to keep trying and trying to get the balloon full, just like someone with an underactive bladder might have to strain and push to get all the pee out.
An underactive bladder can happen for a few different reasons. Sometimes, it's just the way someone was born. Other times, it can be caused by nerve damage or certain medications. Whatever the cause, it can make it hard to use the bathroom normally and might mean someone has to go more often or wear special underwear or pads to catch any leaks.
Doctors can help people with an underactive bladder by giving them special exercises to strengthen their muscles or by prescribing medications or other treatments. With some help, someone with an underactive bladder can learn to manage their condition and still have a happy, healthy life.