a, b, and c are variables but are used as parameters for a function. Like if you had a variable A0 to represent the area of a cylinder and h0 a variable to represent the height. Then, you'd have a function V(A,h) to calculate the volume of the cylinder, where A and h are parameters.
Then say we have V0 = V(A0,h0). Here you see the variables A0 and h0 used as parameters (or crudely speaking, the limitations of the function). Note also, that V0 is a variable but not a parameter because it is not being used as data to calculate something. In addition, you can use functions as parameters, like say we expanded V(A,h) to V(A(r),h) where now A(r) calculates the area of a cylinder's base given the radius.
You can expand it to the following to make things extra clear (though less legible):
operator= (c, operator+ (a,b));
Here you can see a and b used as parameters. Then, the result of the function operator+ is passed in as a parameter to another function, operator=. At this point, c is now passed in as a parameter. So, a and b are parameters while c is a parameter depending on which function you are focusing on.