What is Calculus About?
Calculus is about derivatives and Integrals. Simply as that. Well, explained as simply as that.
Calculus is about calculations that lead to the discovery of the rate of change of a function (i.e. the derivative). The first part of calculus is the derivative, which is the rate of change of a function. The derivative can be applied multiple times, so you can find the rate of change of the rate of change for a function.
This graph of a tangent line is a great example of what a derivative does. A derivative is an equation that can describe the instantaneous rate of change at every single individual point on a function whose derivative is being applied to.