You use the LEVEL with the SELECT CONNECT BY statement to categorize rows from a database table into a tree structure. The LEVEL returns the level number of a node in a tree structure. The root is at level 1, children of the root are at level 2, and grandchildren are at level 3, and so on.
You specify the direction in which the query walks the tree with the PRIOR operator. In the START WITH phrase, you specify a condition which identifies the root of the tree.