SUMX2MY2(list1, list2) performs the following operation on it's inputs.

SUM((list1 - list2)^2)

So, we take the corresponding elements of each list, subtract them from each other, then sum the square of that result.

Example: list1 = 0, 1, 2, 3 and list2 = 3, 4, 5, 6

we'd get (0 -3)^2 + (1 - 4)^2 + (2 - 5)^2 + (3 - 6)^2 = 9+9+9+9 = 36.

Inputs can be lists of numbers or a range of cells.