A derivative is like finding out how fast something is changing. Just like you can find out how fast you are going in a car by looking at the speedometer, you can use derivatives to find out how fast a quantity is changing in math.
For example, imagine that you have a balloon that is getting bigger and bigger. The rate at which the balloon is getting bigger is the derivative. If the balloon is getting bigger very quickly, the derivative is big. If the balloon is getting bigger more slowly, the derivative is smaller.
In math, derivatives are used to study how quantities change over time. They can help us understand things like how fast a car is accelerating, how fast a population is growing, or how fast the temperature is changing. By understanding derivatives, we can make predictions about what will happen in the future, just like how you can predict how fast you'll get to your destination by looking at the speedometer.