Seriously. This is something I struggle with myself. Basically, it means don’t give away too much right away. Secrets and mystery are what keep your reader reading.
For example, in a romance, the secret is whether or not the main characters will be together in the end, or in a fantasy, it’s if they will manage to save the kingdom in time.
Those are secrets that could also be referred to as plot hooks. However, there’s a different kind as well.
Character secrets.
Hidden backstory, some strange weapon, something they’re hiding, all of it counts as a character secrets. Something that’s hidden from both the other characters and the readers. These are like the little barbs on the hook, because often, the hook is cliche. Of course the two leads will get together in a romance, that’s how romance works. But, what is she hiding? How will that affect him? What will he do when he finds out?
Secrets keep the reader engaged, pulling them ever deeper on the quest for answers. Because basically, if a reader can put down your book after the first chapter and not want to keep reading, you’ve done something wrong.
Secrets also make excellent foreshadowing, because we never know just how the secret will affect the plot.
That’s all. Give your characters beans to spill, but spill them slowly. If you spill them all at once, it’s a nasty bean mess and no one wants that.