Calculus is applied math, used for many things from CALCUlating velocity, acceleration, and distance traveled, to CALCUlating Mach or True Air Speed from Indicated Air Speed, air density, and temperature. Much of Physics, Astronomy, and other "hard" sciences rely on Calculus.
The fundamentals of differential and integral calculus are finding rates of change over decreasing intervals, eventually finding a formula for larger intervals.