I watched Hercules the other day and that moment was when Hades walked into the party and delivered some shady opening lines. Good stories aren't always obvious at first, but the moment is sure to come when you tuck in your elbows, squirm a little deeper into your arm chair, and think, “Yisss, this is going to be good.”
This point is the hook. It's what grabs your readers and gives them a good reason to keep reading your book, even if you get a little carried away with a boring description or decide to kill off everyone's favourite character. The most important thing you can do for your story is to give it a good hook.