This graphic shows the order in which Oracle returns the rows for a query of a hierarchical tree. Beginning with the root row, Oracle returns one child row of the root, and then one child of that child row, and so forth until a leaf row (one without children) is encountered.
At this point, Oracle goes up in the hierarchy until the next row with a child row is found, and the Oracle returns that child, and so forth until the next leaf row is returned.
This process continues, with Oracle returning to the root row for the next child only when all of the "descendants" of the first child row have been returned.