The Legendre transformation is a special tool we can use to switch from one way of describing something to another, and it helps us solve problems.
Let's say you have a toy car that can go really fast. We can describe the speed of the car in one way, like saying "the car is going 30 miles per hour". But we can also describe it another way by saying "the car needs 0.0333 hours to go 1 mile". Both these ways of describing the speed are correct, but sometimes it's easier to use one instead of the other.
In the same way, the Legendre transformation lets us switch between two ways of describing things in math. We use it when we want to solve a problem in one way but find that it's too hard or complicated. So we use the Legendre transformation to switch to a different way that's easier to solve.
Like in physics, we often use the Legendre transformation to switch between types of energy. There's a form of energy called the kinetic energy, which is how fast something is moving. And there's another kind of energy called the potential energy, which is how much energy something has stored because of its position.
By using the Legendre transformation, we can switch from one type of energy to the other, and sometimes it makes solve problems easier. Just like how we might switch from miles per hour to hours per mile with our toy car.
Overall, the Legendre transformation is just a tool we use to switch between two ways of describing things in math, like energy, motion, or even more complicated topics. It helps make some problems easier to solve when one way of describing something is easier to work with than another.