Every Cell works with inputs and outputs. Input is the equation that the user is giving to the software, and output is the result of the calculations of the program.
To see how this works, let’s create a new Cell at the notebook and select ‘Free form input’.
As input, type a mathematical equation. For example, the equation of a circle with radius 2 is: x^2+y^2=2^2.
Press Enter to get the output of this equation. The output is the graph of the circle that looks like this:
You can give as input any mathematical equation and Mathematica will turn it into a graph with just a single click!