The controlled system has an input variable and an output variable. Its response is described in terms of the dependence of the output variable on the input variable. These responses between one or several variables can normally be described using mathematical equations based on physical laws. Such physical relationships can be determined by experimentation.
The advantage of creating a system with input and output variables and representing the system as a block is that this representation separates the problem from the specific equipment used and allows a generic view. You will soon see that all sorts of controlled systems demonstrate the same response and can, therefore, be treated in the same way.