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Currying is technically the process in which a function, which takes n input parameters, creates another function, which only takes n- input parameters.   Mathamatically, this is a type of process is called a "transformation".    We can see the process in the "make_addX" example above, where the make-addX function is a function of 2 input parameters when we consider that in order to get a final result that is a number, we must supply 2 input parameters as such:  ((make-addX num1) num2).   See the "check-expect" tests for examples of this.   However, the return value of the make-addX function is a function of only one input parameter. To get a numerical result then, we only need to supply 1 input parameter:

;; two input parameters needed to make numerical result:
((make-addX 5) 3)  ;; --> 8

;; but looking just at the return value of make-addX
(define add5 (make-addX 5))

;; only one input parameter is needed:
(add5 3) ;; --> 8  

What does this mean?  The effect is that the n'th input parameter in the original function, which by its very nature of being an input parameter, is a variant entity with regards to the usage of that function, "make-addX" above, but has been tranformed into an invariant entity with respect to the usage of the returned function, "add5" above.   

(define add7 (make-addX 7)) ;; 7 is the value of the variant parameter "x" for make-addX
(define add42 (make-addX 42)) ;; 42 is the value of the variant parameter "x" for make-addX

(add7 10) ;; --> 17 because 7 is invariant now for add7
(add7 -15) ;; --> -8 because 7 is invariant now for add7

(add42 10) ;; --> 52 because 42 is invariant now for add42
(add42 -15) ;; --> 27 because 42 is invariant now for add42 

One way to look at the currying process is to say that the returned lambda of make-addX "captures" the value of "x" at the moment that it is created.

 All of this is part of the notion of a function's "closure", a topic we will discuss more formally very soon,