Terabithia is the magical, imaginary land invented by the book’s two main characters, Leslie and Jess. It’s a part of a forest and they have to cross a creek by swinging on a rope to get to it. But, once they make it there, Leslie and Jess become queen and king.
The word "bridge" works both literal and metaphorical. At the end of the book, Jess builds a bridge to Terabithia that crosses the creek. It’s an actual object that connects imaginative Terabithia to ordinary Lark Creek. Aswell, the idea of the bridge can also be seen metaphorically, as something that connects Jess and Leslie, that solidifies their friendship, and that links them to that magical place, Terabithia.