What are some of the things that are important in the story?
I guess your question is referring to themes within the story. One clear theme is that of friendship, as depicted between Jess and Leslie, and how good friendship can help us develop us as individuals. Friendship in this novel allows Jess and to a lesser extent Leslie to develop as characters and find their true selves. For example, Leslie's imagination fuels Jess's artistic talents, and it is Jess that forces Leslie to help Janice Avery.
You will also want to look at the symbolic significance of the kingdom of Terabithia - a perfect children's world with no evil. This is an escape for Jess and Leslie, but of course it is also a sight of tragedy, of Leslie's death. Therefore it is suggested that there is a time to move on from innocence and face the troubles of the world, that even penetrate into Terabithia.
Lastly, you will want to think about conformity vs individualism. The novel in one sense is about Jess's search for himself and his balancing of his parent's expectations of him and his own true longing to be an artist. Through the novel and the events within it his father comes to accept Jess for who he is, even though that is not what his father would want.