These are the rules of the game.
When we plug the independent variable into a function, we get the dependent variable. To jog your memory, functions generally have the form f(x). Calculus is all about learning how to pull information out of different functions. We’ll take derivatives. We’ll take integrals. We’ll take limits. We'll even take a few (several) snack breaks. This calculus business is some pretty hunger-inducing stuff.
Functions are super important for constructing graphs; they set the rules for plotting...just not the Voldemort kind of plotting.