The "mood" of a piece of fiction is not hard to distinguish from the "effect" of a story, once you understand what these two terms mean.
In a work of fiction, the mood is what the author makes you feel as you're reading. The next time you pick up a piece of literature to read a chapter, pay special attention to how it makes you feel. Really immerse yourself in the story, pretend you're there watching the action unfold. What emotions are you feeling as you read? Are you happy? Worried? Lonely? Scared? Sad? Angry?
This emotion a novel makes you feel is its mood.
The effect of a story is simply the plot-driven forward motion of it. This starts with the main character's background and motivation. In fiction, one event happens that leads to another, and how the characters respond to that leads to yet another. Each effect becomes the cause for a more serious effect, and the story builds in conflict until its resolution.
That is what's meant by the effect within a novel.