The children keyword signals that the following nodes are considered children of the current node. In VRML 2.0, there are grouping nodes and leaf nodes. Grouping nodes serve to "group" other nodes (its children) together. Leaf nodes signify a tangible thing, whether it's a sphere, a light, etc. The "children" keyword lets the grouping node (in our case, Transform) tell the browser, "Hey, I'm telling you who my children are". In this case, we're about to tell the browser about the ball in the scene, although you could put anything there (whether it's other leaf nodes or even other grouping nodes).