You may need to read the previous article about transformation in order to understand this one.
A scene graph is a general data structure that arranges the logical representation of a graphical scene. It is a collection of nodes in a tree structure, with each node having one parent and a non-negative number of children. Only the top node has no parent.
Each node's transformation matrix is multiplied by all of its ancestors' to generate its world matrix. A world matrix represents an actual position in a scene relative to the origin, whereas a transformation matrix represents a position relative to a parent.
The next article is about 3D.