"There is that ancient tree, whose roots grow upward and whose branches grow downward; — that indeed is called the Bright that is called Brahman, that alone is called the Immortal. All worlds are contained in it, and no one goes beyond. This is that.

In plain words: The world is like an ancient tree that grows upside down: its roots are above in Brahman and its branches spread down into the world. That root is Brahman, the Immortal, which holds all worlds, and nothing goes beyond it.