For each element in A, filter2 essentially "slides" the filter F so that its center lines up with the element in A, and then it does a point-wise multiplication between the elements in A and the corresponding elements in F, and then sums all those products. With a filter that is just ones along the anti-diagonal, each element of the output matrix is found by summing up the input matrix elements along the corresponding anti-diagonal. Eh, it's a bit hard to describe in words. Let's try it with a small A and small F.
A = magic(5)
F = flip(eye(3))
B1 = filter2(F,A)
Where does B1(4,2) come from?
B1(4,2)
It is the sum of A(4,2), the element to its upper right, A(3,3), and the element to its lower left, A(5,1).
A(4,2) + A(3,3) + A(5,1)
In the expression A|1, the vertical bar is the element-wise OR operator. When you OR any value with 1, you always get 1, so this expression appears to be just a tricky way to compute true(size(A)). In Cody, solutions with a smaller code size (based on some metric) are scored higher, so perhaps A|1 had a slightly smaller code size metric than true(size(A)).
