How can I write about the significance of the chapter title "Rulers of Terabithia" in the book Bridge to Terabithia?

The chapter title "Rulers of Terabithia" is significant because it indicates that in that magic land, the rulers - Leslie and Jess - are in control of what happens.  This is important because in their real lives, both children are having a great deal of difficulty because of circumstances beyond their control.  Terabithia, the land that Leslie and Jess have created out of their imaginations, is a kind of haven for them, a place where they are in charge, and where their lives are idyllic, and there is never a need to feel uncomfortable, angry, or afraid.

Jess is the middle child in a family of five.  He has four sisters; the two older ones are whining and selfish while the younger ones are too little to offer much companionship.  Jess' mother is beaten down by hard work and financial worries, and since his older sisters are expert at maneuvering their way out of doing their share of the chores, much of the responsibility falls on Jess.  Jess' father drives a long way each day to his job in the city, and although Jess longs for a relationship with him, he is always distant and tired.  Leslie, who has just moved into the area, is the only child of highly educated parents who have come to the rural town to reexamine their value system.  As such, Leslie is very different from her new schoolmates at the local middle school, and is the object of much teasing and cruelty at the hands of her peers.

It is no wonder that both Jess and Leslie thrive in Terabithia, the land they have created.  There, in the quiet of the deep woods, they have the power to overcome anything that threatens them, and to make sure that things are as they would have them.  Unlike in the harsh realities of their real lives, Leslie and Jess are in charge of the in their magic land; they are the "Rulers of Terabithia" (Chapter 4).