What does calculus describe?
Calculus is a branch of mathematics that describes a certain property through the changes of certain other properties over a certain interval, sometimes using accumulative methods.
For example, a derivative of position with respect to time describes velocity. It could be constant or ever-changing. The integral of velocity over an interval of time through accumulating small chunks of time is then the way the position changes over an interval of time. The area under a graph is the integral.