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 he

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.