Hello, could someone explain to me like a child what exactly is (Change in Magnetic Flux) and how It is applied into this calculator?

 

As simple as it gets. Log into the course and finished all of it. Then you can ask non-childish questions.

https://www.khanacademy.org/science...ic-flux-faradays-law/v/flux-and-magnetic-flux
https://www.khanacademy.org/science...flux-faradays-law/v/faradays-law-introduction

 

In simple terms a field describes a situation where there is a particular vector at every point in space. In the case of a magnetic field this would be the magnetic field vector. This vector describes the force experienced by a charged particle moving through space at every point in space. A vector can change in two distinct ways; it can change its magnitude, or it can change its direction. A "change in magnetic flux" means the magnetic field vector changed either its magnitude or its direction. That's all.

Mathematically, we use vector operations like the dot product, surface integrals, and line integrals to calculate the properties of magnetic flux. We do this because it is a vector filed and scalar operations don't work very well on vector fields.

When you wrap your head around all of this, you should appreciate that there are scalar fields and tensor fields. Each of those has appropriate methods of analysis.