Not quite
If you set level -1 it's infinite, right?
So the first level will be level 0 and so on.
If you have 3 level menu items set on joomla menu and you want to display only to level 2, so on template parameters > Menu > End item level you will set to "1".
In practical terms it will display level 0 (which is joomla level 1) and level 1 (joomla level 2)
Cheers