Ok kiddo, a swim diaper is like a special kind of diaper you wear when you go swimming or play in the water. Normally, when you wear a regular diaper and you go in the water, it gets really, really wet and heavy. But swim diapers are made out of special materials that don't absorb water like regular diapers do, so they don't get heavy and soggy like a regular diaper.
Swim diapers also have a special shape that helps keep poop and pee from coming out into the water. They're designed to fit snugly around your waist and legs, and they have elastic that helps keep them in place. That way, if you accidentally go potty while you're swimming or playing in the water, the poop and pee will stay inside the swim diaper and not get into the water.
It's important to wear a swim diaper when you're swimming or playing in the water, because nobody likes to swim in dirty water. Plus, it's important to be kind to the other people who are swimming with you and to help keep the water clean so everyone can have fun. So always remember, when you're playing in the water, wear a swim diaper!