As @sharky already explained, Joomla's menu's use a nested structure. An excellent resource regarding Nested Structures (how they work and SQL to find nodes, paths, calculate tree depth etc) is available at: http://mikehillyer.com/articles/managing-hierarchical-data-in-mysql/
Edit: The page explains how the lft + rgt in nested structures work. See for example this image (made by Mike Hillyer and shown on his site).
For example: the Television node has the number 2 on the left side (lft 2) and the number 9 on the right side (rgt 9). As you can see, you can indeed use all the "lft" numbers in this structure for ordering. But you can get even more information with lft + rgt, like to calculate how many children a node has:
((rgt - lft) - 1 ) / 2 = number of children.
In this example Television has 3 children: (9 - 2) - 1 = 6 / 2 = 3 nodes.
Joomla uses the nested structures in a couple of its tables: #__assets, #__categories, #__menu, #__tags, and #_usergroups.