Imagine you have a toy car that you can change slightly and still have it be the same car. So, you can move the wheels a tiny bit, or turn the steering wheel a little bit, and it's still the same car.
Now, think about a mathematical function. This is like a magical rule that takes in a number and spits out another number. But just like with the toy car, you can make a tiny change to the function and it's still basically the same function. This tiny change is called an "infinitesimal transformation."
So, if we have a function that takes in x and gives us y, we can make an infinitesimal transformation by changing x just a tiny bit, and seeing how that affects y. We can write this as dx and dy, where dx is the tiny change in x and dy is the corresponding change in y.
Infinitesimal transformations are really useful in physics and other sciences, because they help us understand how things change in a very precise way. We can use them to study things like how objects move and to create mathematical models of complex systems.
So in summary, an infinitesimal transformation is like making a tiny change to a function or system to see how it responds. It's like moving the wheels on a toy car just a little bit and seeing how it still works the same way.