Ha, I actually read his last sentence as sarcasm initially and thought "haven't I heard this joke..." only to find that you had already referenced the one I was thinking of:
"A monad is just a monoid in the category of endofunctors, what's the problem?"
I didn't mean to imply it was trivial to understand, but it's no more difficult than the formal definition of, say, a derivative (which involves a limit, and hence an epsilon-delta construction, which in my experience are not so easy to fit in your head at first). Understanding what a multigraph is should be doable.
"A monad is just a monoid in the category of endofunctors, what's the problem?"