Coraline slips into some kind of parallel version of the new house she has moved into with her parents, where there are copies of everything and everyone that she knows of in the real world, even of her mother and father, who are called 'her other mother' and 'her other father'. But there is one big difference: each copy is some sort of distorted mirage with black button eyes of the real version, and everything there is much more exciting, but also much more scary. And one of these things has kidnapped her parents.
She has to go through several creepy encounters with those monsters to save her parents. While doing this she is far from being fearless, she is actually terribly scared. But Coraline summons her bravery out of love for her parents and out of her desire to save them and that is what makes this book so creepy and beautiful at the same time.
And it also gives us the important message, that being brave doesn't mean you aren't scared. It means you are scared and do the right thing anyway. Something that author Neil Gaiman wanted to tell his daughters, for whom he has written the book.