ORDER SIBLINGS BY

The rows in a hierarchical query are returned as a tree, the children following the parent. ORDER SIBLINGS BYpreserves the hierarchy and orders the children of each parent.

SELECT
   CONCAT
   (
      LPAD
      (
         ' ',
         LEVEL*3-3
      ),
      ENAME
   ) ENAME
FROM
   EMP
CONNECT BY
   PRIOR EMPNO = MGR
START WITH
   MGR IS NULL
ORDER SIBLINGS BY
   EMP.ENAME;
ENAME
----------------
KING
   BLAKE
      ALLEN
      JAMES
      MARTIN
      TURNER
      WARD
   CLARK
      MILLER
   JONES
      FORD
         SMITH
      SCOTT
         ADAMS

Clark comes after Blake and before Jones; they are under King and ordered by their name. Their children are sorted and the hierarchical appearance is preserved.
ORDER BY without SIBLINGS destroys the hierarchy:

SELECT
   CONCAT
   (
      LPAD
      (
         ' ',
         LEVEL*3-3
      ),
      ENAME
   ) ENAME
FROM
   EMP
CONNECT BY
   PRIOR EMPNO = MGR
START WITH
   MGR IS NULL
ORDER BY
   EMP.ENAME;
ENAME
--------------
         ADAMS
      ALLEN
   BLAKE
   CLARK
      FORD
      JAMES
   JONES
KING
      MARTIN
      MILLER
      SCOTT
         SMITH
      TURNER
      WARD