I already read the calculus book and I only understood that calculus is used to get differentials and integrals.

Spreads are a derivative, which is the rate of change of finding the steepness of a function.

The integrals are the sum of squares to find the area of an object that does not have a formula.

It also has to do with finding the middle of a function curve and finding things like the local minima and maxima of a function.

What else did I miss?