"Explanation", and anyway, this is redundant, since that's kind of what the "Notes" section is for.

However, I still don't understand the explanation. Often, an example is worth a thousand words. So, I would replace this with an "Examples" section, written in doctest format (see other Examples sections in scikit-image to see how it's done). Write an example with a (5, 5) 2D image integrating from (1, 1) to (3, 3).