what is the meaning of life?
seriously, calculus is the science of reasoning backwards from a knowledge of the forces that cause things to change, to an explicit understanding of how those things evolve.
for instance, from a knowledge of how the pull of the Earth causes falling objects to speed up as they fall, one obtains an explicit formula for how far an object falls in a given amount of time.
from a knowledge of what forces govern population growth, one obtains a formula for the number of members of that population 20 years in the future.
the fundamental theorem of calculus says that the rate of change of area under a curve, as we move to the right along the curve, is measured by the height of the curve. hence froma formula for the height of acurve we obtain a formula for the area under the curve.
there are many more applications.