Nappies are special pants that babies wear to help them not make messes everywhere when they go potty. When babies are little, they can't control their potty very well and they go pee and poop a lot. Nappies help catch their pee and poop so it doesn't go on the floor or their clothes.
Nappies are like disposable underwear that you wear for a little bit and then throw away. They have a soft, comfortable layer on the inside and a outside that's waterproof so the pee and poop don't leak out. You put them on the baby by sliding them up their legs and then fastening them around their waist with special tabs.
When the baby goes pee or poop in the nappy, it gets absorbed by special material inside the nappy. Sometimes the nappy gets really full and starts to feel heavy, so then it's time to change it for a clean one.
Sometimes people use cloth nappies instead of disposable ones. These can be washed and reused, which is better for the environment and can save money too. But they can be a little bit harder to use than disposable nappies.