A natural disaster is when something very bad happens because of nature. Imagine that nature is like a big box with lots of different things inside that can be good or bad. Sometimes, some of these things can mix together in a way that makes bad things happen.
For example, when there is too much rain, flooding can happen and water can go into people's homes, making them very wet and damaged. Or when there is a big storm like a hurricane, it can be very windy and cause things to fall down, like trees and buildings.
These natural disasters can be very dangerous and make people sick or hurt, and sometimes even cause death. That's why it's important for people to be prepared and safe during these times, and also to help each other out when something bad happens.